What changed?
My stepson is 11 and I have been in his life since he was 5. He took to me right away. We were super close right from the start and he was even calling me his stepmom when my husband and I hadn't been dating for maybe 6 months. (we told him to just call me by my name at the time to avoid drama with the ex and you know just incase i didn't turn out to be his step mom...luckily I did 
)
At the time, my husband had 50% custody so I saw my stepson several days a week and both me and my husband were very active in his life. Fast forward two years and my husbands ex decides she's going to move to another state with her fiance. Long story short, she takes stepson with her and now we only have him on summer breaks, spring break, and rotating holidays. We've been on this schedule now for 3 years and it's still just as hard, if not harder.
When they first moved, my husband and I's relationship with stepson stayed strong. We were worried that our bond would fade, with us not seeing him as much, but it didn't. Every time he was with us, it was like we picked up right where we left off.
It wasn't until about a year and half after they moved that it started feeling like with every visit, our family dynamic felt less and less strong.
Stepson just seems different and when he's here to visit he seems distant and I hate even saying this, but sometimes it feels like I have a stranger living in my house.
The three of us used to be so close, and stepson was so loving and affectionate...now sometimes it seems like he doesn't know how to act around us. Like he's uncomfortable? I don't know. I love him dearly, and my husband does too. I know it tears my husband up that he doesn't have as close of a relationship with his son anymore.
I need some advice 
pre-puberty could be kicking
pre-puberty could be kicking in?... it doesn't hurt to have your DH talk with him and try to get a handle on what might have changed.. maybe it's that he is starting to have more meaningful friendships and it's harder for him to leave now?
I do often wonder if it's
I do often wonder if it's just him getting older and maybe starting puberty. And I am definitley worried because I know that there is going to be a time, if it's not already here, that he doesn't want to leave his friends.
Is BM the type who could be
Is BM the type who could be trying to turn SS against you guys?
I don't want to think she
I don't want to think she would do that, but honestly I don't know. She was very against me being in stepsons life in the beginning of mine and my husbands relationship. She didn't want to meet me or recognize me as a role in stepsons life...not until she began a relationship with her now husband. She changed like a lightswitch and all of a sudden wanted to meet me and started to be nice. Aside from the very beginning, there's never been any negativity between her and I...we actually don't ever really speak and hardly every see each other. I don't think she would try and turn stepson against us, but at the same time I don't think she really encourages the relationship between him and us either. After all, she did move to another state knowing that it could strain the relationship.
Why did your dh agree to let
Why did your dh agree to let her move and take ss? (Or was there a court battle?)
It wasn't easy...
He wasn't given much of a choice. There were lawyers involved, but I'd like to leave it at that. It was a rough time. Sometimes we have to make decisions that hurt us, in an attempt to make things as easy for the child as possible.
My YSS at about 11 stopped
My YSS at about 11 stopped being affectionate. No more hugs. No more cuddles on the couch. No more tickling or play fights or whatever. He just didn't want to be touched and didn't want to touch other people. Nothing that we know of happened to him other than puberty started hitting him like a freight train.
Him and OSS withdrew from DH and I (and BM - that's a whole other story) once they hit their preteens. They were just more interested in friends, hobbies, alone time, etc. Two kids who would be up any adults rear end asking for them to play games or watch a movie with them disappeared into their rooms never to be seen or heard from again (until the dinner bell rang).
It's likely time for DH to have "the first of many talks" with him about life, puberty, what he's feeling, etc. If he and BM are on speaking terms, he may want to consult her as well to see if she has noticed the shift.
Remember, he is growing up and won't stay the same sweet, affectionate young boy you remember. And that's good! The teen years are rough, even when they're easy. Giving him space to be able to process his life is a good thing.
THIS!
THIS!
This is the change that comes
This is the change that comes with puberty. He will be fine with you. Try to remember his age, when he has mood swings.