I've been with DP over 4 years, known DS (currently 11) for last 4. I've tried so hard to get even a small bond there but cant.
Firstly, he has absolutely no social skills whatsoever. Not just with me. With anyone. You ask him a question and it's one or two word answers - so 'what did you do on your hols' is met with a shrug and 'not much' or similar reply. He never says Hi or Bye - gets here takes his stuff up to the bedroom and stays there til dinner, comes and eats then goes back to his room. There's no attempt at social niceties or courtesy and any attempts on my part to engage with him just dont work. He's like this at his mums and my IL's too, although can be slightly more chatty - not much though. The only time I hear him chatting is when he's playing on the games console with his friends. (And before anyone suggests social communication disorder such as asd/aspergers - I've thought of that but having worked in SEN for a number of years he doesn't display any of the other markers so it's unlikely)
To combat the issue of him sitting in his room all the time we've tried to engage him in activities - bringing him on trips out with and without my kids (DP also offers to take him out without me so he has 1-2-1 time with Dad) but he either refuses to go or comes along but doesn't get involved or shows his displeasure at being there. Even when the activity has been his choice. E.g on the last bank holiday weekend I asked what he'd like to do, he said paintballing. So I booked paintballing for the kids. On the morning he refused to go. When DP told him he had to, he went along but just walked around sullen and miserable refusing to take part. We've tried doing things at home with him as well - card games, movie nights, water fights etc but he has no interest at all.
So as not to drip feed - I will be clear that in the early days there was some problems with him accepting me. He'd never known his Dad to have a partner (DP and Ex split up when he was 1, we met when he was 7) and I expect that it was difficult adjusting to sharing his Dad. He tried to manipulate things, lying to his mum about things here and refusing to come round - at one point he tried to tell DP that if he didn't split with me he'd never go to his Dads again. His mum thankfully knew what was going on and encouraged DP to not give in to him.
I do wonder whether his attitude here is sometimes a grudging acceptance that his Dad has a partner and there's nothing he can do about it. But then I see how he is elsewhere and realise that (and I'm sorry how this sounds) he is actually just a sullen moody child who unfortunately hasn't yet developed much personality. I dont dislike him, I'm always kind to him, involve him and treat him as I would my kids but there's just no connection there at all. Is it possible one could ever develop? Or if it's always like this, whats the likliehood of it having a long term affect on all our relationships? (Him with his Dad, me with DP etc)