If or when you move in SS is not allowed in your bedroom.  Don’t you two do thing in that bed, Don’t you want time alone with SO.  Are you NOT going to lock the door at night ?  You don’t want SS to walk in and see something going on.   Get Real,  your bed room is your bed room. Not a family room.   Have SO move the kid and his junk out of that room right now. Or you should move out right now. OR RED FLAG

Advice from someone who made the mistake of moving into their home. Don’t do it. Start fresh. Own your territory. Make it known things are starting fresh. No skids in the adult bed.

I couldn't deal with that for even one second.

Because if you can't discuss something as major as where your future SS is going to sleep, you are not ready to get married. This is something you need to share with your fiance, "...but I get a little jealous knowing that majority of the nights he will be snuggling him the whole night and in the morning and not me." So are you going to be sleeping in another room? How do you handle it now?

I can't believe you are getting married "very soon" and no one has discussed what will happen in the bedroom!

Let's just say you are wrong for bringing it up so you don't. How do you think this will play out? Do you think in five years you'll be sleeping with a 10-year-old and your husband in one bed? How will that look in 10 years?

What if you bring it up now and your fiance tells you this is not negotiable -- the kid stays in your bed? Don't you think it's important to know your future husband's boundaries where his kid is concerned BEFORE you get married? What are you waiting for? Do you want that kid in the bed on your wedding night???

If you can't discuss this stuff with this guy, you had better rethink getting married to him. Marriage is not easy even under the best circumstances. I would be worried that your fiance hasn't brought this up yet. It tells me no one has discussed the most problematic issues of marriage: stepkids and boundaries. It tells me everyone is assuming the other person will back down or not raise issues that clearly must be addressed at some point.

When you bring this up, that would also be a good time to ask if the kid will have a bedtime, if you will be responsible for driving him to school and other events, if SS will be coming on all your vacations, if alone time will be carved out just for the two of you, if financial decisions regarding the kid will be made by both of you or just your husband...

You better get talking so there are fewer surprises when you're married because once you're stuck, you're stuck.

This is a BIG area to discuss now. Please don't wait. 

I also agree that moving into your SO's home might not be  the best plan. Is there anyway that you guys can find some place new for your new life? 

Chances are, you are going to find your SO getting mad at you for this. He is going to say it is no big deal, it hasn't bothered you before, you never had an issue before, so on and etc. This should be a giant red flag on what is to come. You aren't going to matter in pretty much anything. I only say this becasue he has proposed to you, planned on you moving in, and still hasn't condsidered you in sleeping arrangements. 

Having a child sleep in their own room and own bed isn't wrong, it is healthy and wanting a private place for you and your SO is NORMAL! Don't let him tell you anything else. Not having that boundary is where the issues come in. -- This is all from a bio mom of 2 and SM of 2. Kids don't step foot in me and DH's room. Ever. 

You have every right to claim that space as just goes and your husband's.  But you need to do it BEFORE you move in. You need to talk to your husbsnd NOW. have him start implementing it NOW.  That way the kid is used to it before you move in.  Also that way the kid doesn't associate this change as because you got married.  Because then he will forever blame you for it

SS needs to transition to his own bed BEFORE you move in, so the change is not "your fault".

You definitely are not wrong for wanting the bedroom to be your space. Have you not slept over there already? If so, does SS sleep in the bedroom when you sleep there?

Like everyone else said, definitely don't let him sleep in the room when you are there. He is not your bio son and unfortunately, it could be perceived as inappropriate. You are lucky it seems you don't have a BM to deal with at the moment, since usually they are the ones to accuse SM's of being inappropriate and call CPS...but still. You need your place as a couple, and nothing will take the shine off being newlyweds faster than a 5yr old sleeping with you.

Second: It is pretty concerning that you haven't discussed this already with your SO, for several reasons. First, it seems like things have been pretty smooth sailing so far regarding the skid. You're lucky, that's rarely the case. But, this is the exact type of situation where issues start to present themselves. Bio dads are typically VERY defensive of skids, and don't really like SM's to change things. You may very well see a whole other side of him when you present this. And it could just be the tip of the iceberg. For your sake, I hope he is open to your suggestion that skid sleep in his own room and things continue to be great, but you would be an anomoly. Most of the ladies (and guys) on this site have had the opposite reaction when it comes to any suggestions or perceived "criticism" of their parenting, and have been met with anger and defensiveness, no matter how logical or in the best interest of the child the suggestion is. Either way, pay VERY close attention to his reaction when you bring this up (and you 100% should bring this up). If it is negative and defensive, that is a red flag, and this will not be the last you see of that attitude when it comes to parenting, and you should seriously consider if you can handle that type of conflict. If he is receptive, then great! You are lucky and hopefully won't end up dealing with super protective, stubborn DH's who fight you at every turn when you try to provide input. Good luck!

Let me tell you how this works out when nobody discusses it first, because my stepson's mother is going through it now. My stepson is 6 and has always coslept, calls her bed "our bed", keeps his junk in her room, did not even have his own room until a month ago, you get the picture. 

The kid becomes confused and traumatized when a new person is there and expects to sleep in what they think is their place. They cannot sleep and camp outside the parents door. Nobody gets any sleep. My stepson mother's boyfriend moved out and broke up with her because of this. He came back but it is still constant power struggle between her boyfriend and the kid. For some time, she would sleep in his new bedroom with him and the boyfriend who does not have kids of his own would sleep alone. This is not the way anyone pictures their life to be when they move in with their SO.

I agree with those who suggest moving to a different place. With my stepson, we know he can sleep in his own room becuase he does it at our place. It has never been an option to sleep in our bed. He does not think our room is his. (My husband allow him to walk into our room at any time and I am not ok with that, but that is a different topic.) The only place he knows to sleep in our home is his own bed. My husband even advised his ex-wife, when she and her new boyfriend moved to a new flat that they make the rule that stepson sleep in his own room, that's how it goes in the new flat. She did not make this rule and they have the same problem as they did in the old flat. This even caused problems in our home. He always slept ok in his bed at our home, but when the boyfriend moved in at his mother's home, he kept us awake all night, would not leave our doorway, and clung to my husband worse than a baby. It was awful and I was so angry with them for not making any preparation for this big change and leaving us to deal with their mistake (we have 50/50 so lots of back and forth).

No matter your situation or your options, the one thing I will say you must do is have this conversation now. It is like for people who are pregnant with second child, it is advised to hide all baby things from the first child many months before the new baby is born so they no longer think those things are theirs and that the sibling stole them. Put some time between the kid being moved to his own room and you moving in so he does not associate the two things so closely. I can tell you from personal experience that the SO moving in and the kid being moved from the master bedroom at the same time is disaster at same age as your stepson. 

Red flags are waving all over this post. 

OP do not marry, do not move in until your Fiance has solved the problem He made. 

You sound like a lovely person and I'm not sure you know what you're getting into here