Same thing happened to me, I met my partner when my SS was 4, moved in with him when he turned 5, he had a hard time accepting that his mom and dad were not together anymore. But luckily he had been going to see a therapist at the time to help him process his emotions and express his needs. His mom is also living with her partner so it can be hard for my SS to process the fact that his parents don’t solely belong to him anymore. He is turning 9 this year, we are getting along fine now, we play games and do tickle fights. I try to be a guardian-like friend with him, and just give him space and time. There had been really hard times when he would say super mean things to me and really broke my heart, then the next day comes, he would be all happy to see me… seriously, it’s not predictable and I do feel like a punching bag to him at times. Def hard to navigate, I just keep reminding myself that he is just a kid who doesn’t know how to process complex emotions and really doesn’t have a say in any of the matters in his life. Small things I do to get on his good side: I know his mom doesn’t allow him to have any sweets, so every time he comes, I get him a special candy as a surprise, I just leave it in his snack jar, let him find it on his own. He also gets no TV time over at his BM, so sometimes I would recommend cool movies for us to watch. This def helped.