You cannot control how BM parents, or what goes on in her house, so don't even try. Children can figure out and understand that the rules are different in each household. There is no reason for you to "carry her weight," and if anyone should be making up for her parenting deficiencies, it should by your DH, not you. If she chooses to ignore her son, it will have consequences on their relationship, and that will be on her. You and your DH should parent how you see fit, and you should never let your worries over your SS take you away from your own child.

A piece of unsolicited advice - you will be much better off if you think of, and refer to your step-son, as your "step-son," and not your "son." He already has two parents. And while you may not think BM is a very good parent, she is his mother and she is going to mother him in the way that she chooses. You can't care more than she does, it will do you no good. Read around this site and you will see in the vast majority of cases that kids always choose their biological mom over their step-mom, even if the bio mom is not a good parent.

Why would you have issues regarding closure with your DH's XW?  That is not your relationship to have closure with.

Odd. IMHO.

Your pespective that BM needs to prioritize her son over her marriage is dead wrong. Kids are not the priority in adult relationships. The marriage and partners are the priority for the equity life partners in the marriage. If they are not, the relationship is on borrowed time.  Kids are the top relationship responsibility but not the priority.  In all liklihood the falacy of kids being the priority in adult relationships is what ends adult relationships. Particularly blended family marriages that include   at least one failed family veteran.   IMHO.

As for your dictation to BM how she parents and structures her relationship with her son, that is none of your business and If our blended family opposition had tried to pull that shit it would have been game on.  

Fortunately for us.... they were toxic, manipulative, and petty.  They also are in a dead tie with an Amoeba in intelligence and resources so we could keep them firmly in their place and under control with a rolled up copy of the CO to beat them about the head and shoulders with and by applying the CO, supplemental county rules, and state regulations.  When I say they are in a dead tie of intelligence with an Amoeba, I am including at least three generations of that shallow and polluted gene pool collectively.

IMHO you need to back off.  Set the standards of behavior and performance for your home, hold your SKid accountable to delivering to those standards when he is in your home, and quit fantasizing about some relationship with your DH's X that you should not even be considering.  Keep SS abreast of the facts so that as he matures he will be able to understand healthy relationships and be able to protect himself from manipulative adults in his life and protect himself from them  into his adulthood.

Again... IMHO.

Good luck.

My best advice is to take a giant step back and put BM on ignore. You don't need to be friends, you don't need a relationship (civility is nice though), you can't control what goes on in her house. It would be a lot easier if the parenting styles were similar at both houses, but they are not and you can't change that. You're wasting energy trying.

Where is your DH in all this? If you are tired, then he needs to step up HIS parenting. I fear you are overfunctioning for him, and trying to overfunction for the BM too.

You don't talk a lot about your DH and how he parents.  Clearly, he had no problem with how things were proceeding before you came on the scene.. so he is just as "guilty".. it seems of not being a more structured parent. 

I will say, that in your post, you talk a lot about your Skid as "my child".. and that, in itself, can cause conflict with a BM.  She is really under no obligation to coordinate parenting with you.  And coparenting doesn't even involve you really.. you are not that child's parent.. he has two a mother and father.. and it's up to them to navigate that.  And coparenting doesn't even mean you necessarily have the same style of household either.. or rules.  Children are able to navigate and understand that rules and expectations of behavior may be different... and trying to impose your standards on her home IS overstepping.  And it is an exercise in futility.. so you do need to adjust your expectations of what you can and cannot control.  You can't control his EX or what happens in her home.. unless it is outright dangerously harmful to the child (then call CPS).. her allowing a less structured environment for her child doesn't necessarily make her a bad mother.

You may or may not be the only root cause of the child's improvement either.. the child is older.. has had more school.. more time to mature and develop... so time is often a great healer right?

I appreciate that you are putting your heart and soul into this child.  But, I think you will end up happier when you stop trying to control what happens with his EX and stop trying to have a relationship with her.  It isn't necessary really and likely is a point of conflict anyway.

BTW.. the meeting that you called with her early on?  It was not her place to explain the logistics of her separation from your BF.  She maybe even thought you knew.. making your meeting even more awkward for her.   I don't think you need closure with her because she didn't decieve you.. it wasn't her relationship with you to be honest about.

I feel for you, you seem like a genuinely caring and loving person. It is really a shame that BM has ruined all efforts at coparinting and is doing such a lousy job at parenting. I'm in a similar position as in my case BM allows my SS to do whatever he wants resulting in obviously conflicts with discipline and expectations of behaviour when with us. 
I can only advice, and this is something that others have advised me here, to set your rules and structure in your household and forget about whatever happens at BM. Unfortunately you cannot do anything about that and having any expectations from her will only frustrate you. You will have to let it go (I know, easy say than done)

good luck 

Maybe it is wisdom.

You are new here and relatively new to the stepkid situation. There are many members on here who have been doing this for decades. 

One area you seem to be challenged with is the multiple times people have told you this is not your son. You disagree and feel biological parenting roles are "archaic" and not "black and white." The fundamental human connection is between children and their mother. The mother-child bond is the strongest. Always. 

Consider this an area where you need education and an honest assessment of your own role. Is it possible your stepson is conflicted because you have stepped into the role as his mother?  He has a mother, it's not you. When others have pointed this out you become very defensive. As a social worker, and mother yourself, surely you understand the strong biological bonds between mother and child.

If your biological child's stepmother essentially pointed out all your problems/mistakes as a mother, what would YOUR response to her be? (Imagine BM telling you, "I wanted to point out some issues with you. I think you are spending too much time focusing on other people's parenting and you are ignoring your own child's needs. As a mother, shouldn't you be making your own biological kid your priority?" 

I am glad that you are seeing a therapist and hope you further explore these issues during therapy. Please be honest with what your thoughts are about your relationship with this child. Maybe print off this thread and your defensive responses to others. I think it will be enlightenting for your therapist.  And you.