I have a 12 yr old DS and last year we moved in with my partner of 2 years. DS is an only child and as I have no family, we have been quite insular, through nobodies fault. However, he has had regular contact with his Dad, although this hasn't always been as consistent as I would like.
I suppose what I am worrying about is that DP has 4 children and I feel that DS is a completely different child around them. He is the easiest boy on the planet when it is just us two, but he becomes "hard work" when the others are around, I don't see him at all as he is always off trying to keep their attention and it's impossible for me to talk to him alone. I've been having lots of doubts about whether moving in was a good idea recently and DS behaviour does bother me. I understand that he might "show off" in front of friends etc., but should he really still be doing it in front of children who he spends most of his free time with 8 months down the line?
I am putting in place some time for him and me to do things alone to try and help him. When I talk to him about it he says he is quite happy and loves having DP kids around, although he does things like hide his things (because they take his stuff without asking) and generally tries to keep the other kids attention all the time. Sometimes I hear the DSC trying to put him down and tell him that they are better than him and he just takes it, the only one he won't take it from is the little one aged 8. When I ask him about it, I tell him to not be afraid to be himself, to stand up for what he thinks is right and he is not here to entertain them, there is nothing wrong with having his own space and time without them. I would like to see him put in some boundaries so they don't just see him as the joker and the one to entertain them when they come. DSC all seem quite happy to not be joined at the hip to each other, but DS seems to always want the attention of one or two of them, at least.
Also, I find myself knowing a lot about DSC lives and have little chats with them about stuff. DP doesn't seem to engage at all with DS even though they do get time alone together when DSC aren't around. Again, DS doesn't seem unhappy with DP relationship but surely it should be better? Is it just that men are a bit rubbish like this? To be fair, I don't think he understands his kids school reports or knows all their friends names or where they live etc. I think I would know more about that for his children, than him. My ex has never really shown much interest in DS or done anything nice with him and I just wonder if DS thinks its normal? I don't want him to feel like that but I can't make his Dad be any different or his step Dad, it seems.
So, am I worrying uneccessarily?
I also wonder if I am just sad because I feel I had a lovely one to one relationship with DS before we moved in and I feel it has been taken away in one foul swoop. I feel a bit like I have lost my child and gained DP's children, and although they're pretty good kids, my priority has to be DS. 
I also think I am / have been guilty of putting other peoples needs before my own in the past and wonder if this is what DS has learnt from me?