He's not ready for a grownup relationship because, as you have observed, he is WAY too enmeshed with his "ex". They are still very emotionally involved, TOO emotionally involved.

No wonder his son has attachment issues - he doesn't understand what's going on. His parents are behaving as if they were still a couple yet they don't live together and now you've come on the scene. Poor kid is completely lost.

As long as he and his ex are as emotionally involved as they are, there will be no place in his life, in his world, for you. You will find that he's probably very generous (guilt gifts) but that you're relegated to a status of being his girlfriend but not his confidante. You will be trotted out for show, be expected to be his on call bed warmer, possibly also his maid, nanny and cook if you move in.

FWIW, he KNOWS that he shouldn't be best buds with his ex - why do you think he has a panic attack when you bring her up? It's because she's the "other woman" and he knows it's wrong but he just can't help himself.

THe worst in all this is that there's noting you can do to change this. He may pay lip service to your requests but he keeps on going back to BM - they're BFFs.

Dammit, if they're so happy chatting and being together (do they do joint Xmas, birthday, etc. celebrations? Bet they do) why don't they just get together again and avoid making other people unhappy? I'm inclined to say to throw this one back. Normally, I'd say to try counselling, but, in this case, I'm not sure it would help much because he's clearly happy with the status quo. Maybe go by yourself to sort things out as regards your future?

He is making you "the mistress". 

They are very much emotionally tied. He is not ready to be in a relationship with someone besides BM.

Quit wasting your time.

People say it all the time: When a man shows you who he is, believe him.

This man is showing you that he has a primary woman in his life - his ex-wife.

You are the woman he needs by his side to meet the needs she is no longer willing to provide.

A lot of couples have a period of being enmeshed after separating, but it is usually temporary before they work out how to move on without being in each others pockets. This guy is still in this stage 5 years after his break up....this relationship has 3 possible outcomes:

1. Nothing changes BM continues to get your boyfriend to do all her odd jobs and she is the third party in your relationship.

2. He changes with your guidance and BM is put out so decided to make your life difficult because she resents loosing her bestie to you. Your instillation of boundaries on the boy are also taken negatively and you get blamed by both BM and ss for ruining their lives and they cause chaos for you. 

3. you leave this guy now and move on with your life finding someone who is not still spending half their life running after their ex. 

You said you didnt think youre ready to move in with him.  I dont think he's ready.

I agree with all the other posters above.  Another way of putting it is: has he done the work?  In the step world, that means has the father established his own home?  Does he have the child's bed and other nevessary furniture, like refrigerator?  Does he have a sensible routine set up for the child with reasonable expectations and parenting, ie, not Disney Dad.  Here is where your SO starts to stumble.

It also means having a business-like relationship with BM including a legal custody agteement with child support spelled out.  Him not having this is a huge red flag.  He is also way too enmeshed with BM instead of moving forward with his own independent life.

I lived with this type of thing for a few years.  DH had huge daddy guilt and BM was very dependent and manipulative.  Major bad memories are him rushing over there on our first Christmas morning, him buying her a car and the endless calls for every "emergency".  He had his child support spelled out but went way over and beyond and it never stopped til all 3 kids moved in full time.

If I were you, I'd move on.  Save yourself some grief.    He's just not ready.

DO NOT move in with him. If you aren't wanting to go ahead and end it, at least don't mix finances or living arrangements. I am so glad i didn't. The thing is, a guy who continues to live like this after 5 years probably is lacking in other ways, too. My SO was like this with BM2 10 years after their split. BM2 finally went too far and their enmeshment ended, but i found that my SO's lack of boundaries and poor decision making extended into just about all areas of his life. Once this problem is beaten, other things will show themselves. This guy is trying to convince you that he is life partner material, when the evidence states otherwise. This is a form of dishonesty and it's part of his character. BM enmeshment is what you see now, but probably the tip of the iceberg. 

He has to cut ties with. BM. With texting what has a record of what the text said .. There no reason to talk with BM.  and the day he left me to help out BM and her care Would be my last day with him.  You can't live this way, you can't be fourth in a relationship,  you can't have BM controlling your life.    RUN. RUN. RUN 

This is how they operate and that's going to be hard to change, since they obviously are okay with being so enmeshed. You are going to be the bad guy for trying to change their dynamic.

I think that's really important: SHE ended it. He wanted it to work. These boundaries are too weak regardless of who ended it, but at least if he had initiated the divorce he could look you in the eye and say: If I wanted to be with her, I would be with her. He can't be with her because she didn't want it. That means you really can't know whether you are second choice and the fact that he isn't changing the dynamic with her despite how uncomfortable you are doesn't help.

Being the backdoor woman in your own relationship makes no sense.

Get on with living your best life and leave this failed but overly ingrained former family to their ongoing disfunctional trainwreck.

Sleep in the same bed for the kids. 
I could never be in this type of relationship. When I first got together with my DW. She took the kids to there fathers her ex. Giving me some BS story that se spent the time with a neighbor, not with the ex.  I told her. She could do whatever she wanted. But not with me. The ex can pick up the kids if he wanted to see them. He could pay for the gas, the entertainment , the food ect.  

I wasn't paying for her to take the kids to the ex's  I was not playing Happy Family with the ex.  I really don't know if she was relieved, had the guilt removed. It became my fault  Not hers,  it was never throwned in my face .  But I could never live that way. Ex has to be out if the picture.  Today everything should be in saved texts,  Only discussion is about the kids, pick up time/place  return time.

He really isn't worth the trouble ... This is the perfect illustration of "love is not enough".

I would let him go and play his games with his ex - he's made it clear he won't change. Ask yourself what he's really getting out of this situation. Chalk it up to experience, at least you won't be taken for a ride next time.

I agree with everything that's been said. He's still in a relationship with his ex, so he's not emotionally available to you. I had similar issues at the start of my relationship with DH. In my case, he was the one who left and felt like he abandoned her, so he was "on call" a lot. I told him that I understood why he felt responsible for her but made it clear that I wouldn't be able to tolerate my partner putting another woman's needs ahead of mine. I told him that I wouldn't blame him if he felt he had an obligation to her, and that I even respected him for wanting to ensure she was okay, but I wasn't going to be a part of it. Luckily for me, he put boundaries in place. Lucky because, similar to you, I was head over heels in love with him and it would have been very hard to walk away from him. And that's what I wanted to say to you. It feels good to be validated in your feelings and it gives you motivation to act when people tell you to move on. But the actual "moving on" is much harder. It's not easy to find love in this world, so if you have to move on (and I believe that you do if he won't set proper boundaries) be kind to yourself. Let yourself mourn what might have been a wonderful relationship if you'd met at a different point in your life. 

Or else we would not be here.  No one is saying "my life is great".   That I met my SO  and it's cream and strawberry's.  We all have problems. You are not alone.  It better to not to wast your life.  With some one who rather be with someone else.   
These people divorced, knowing it will screw up there kids, then spent the rest of their life trying to unscrew there kids. 

Im late to this Game, but wanted to comment:

When husband and I first met each other I thought it a little odd that he was "helping" his ex move a few different times (hence why he missed a few outtings). Then we started our relationship, knowing that he was separated-not-divorced and even though Toxic Troll BM had a partner (short lived) Tweedle Dum, she still did what I call "The Ask". Husband bought 2 diff beds for kids in her apartment, dropped bags of groceries off, etc.

Finally he got a divorce, and gradually less and less "asks" happened.

He did it, he said, for "he childreeeeennnnnn". 

Thankfully you had the strength and wisdom to get out before you got stuck. Im stuck (we married, bought a house...) and am gathering my strength to do the Big Goodbye, but thank you for updating and staying on here - you are an inspiration (and you ROCK)

Playing house with you, and happy family with the ex.  Having two woman fighting over his attention.  This man needs mental help.  First of all he must understand what divorce means,  it's not good for the kids, but it's there choice to divorce,  

You want a relationship with a man who not in a relationship with someone else..  understanding once you have sex with a person you can not go back as friends.  You can not sit by with co party's, co vacations sharing a room. They may be nuts but you are not,  

'Better be along then watch this dysfunctional way of life. 
'I personally would not allow this, and did not allow it. SO wants a relationship with me, they must cut off the relationship with the ex.  No co anything. Holidays, birthdays, vacation.  Graduation and wedding are the two co events. Allowed.  A person must commit to me as they committed to the ex.  They want to have a relationship with me, then nobody else is in that relationship.  When they were married to the ex. They didn't have other people in the relationship. Did they?  
'People can live, and do as they like, nothing is right or wrong in someone's eyes. What ever was going on with tge ex was dysfunctional. He can't act the way he was and expect someonecekes to stand for it b

If they divorced, it's going to screw up the kids.  There originally happy family, what not that's happy if they are divorceing. Is gone. The joint holidays, birthdays party's, are finish for.  Expecally if they want a new relationship,  No new partner will want to be second to the ex.  No new partner  will want to share life with the ex.  
'This  is a discussion the divorce people must make .  Stay single and play house with the ex but live alone. Or cut most ties  with the ex and start  a new relationship.   
'For the kids mean DR appointments, school stuff. Sports, CO schedual.  Not joint BDAY parties.   Today communion with the ex should be done by text so there copy's of it. The So should have access to read those texts.  You can not be friends with the ex. Once you have sex with someone you are lovers, can't go back to friends.  I sure your SO doesn't want you to have long telephone calls with your ex lovers.