Pre teen help....please 🙄

My husband recently got full custody of his two children, 11 (girl) and 12 (boy). I know-crazy that the dad actually won custody! We are so thankful. It’s been about 2 1/2 months of them living here full time. My step daughter is incredible and we’re best friends! However, my step son is a different story. When he comes back from moms house, he’s a totally different kid. Horrible attitude, lazy, and he’s been telling different stories to mom and dad lately. He’s the one who told us he wanted to live here full time and asked us to make it happen. So we did. Now he’s telling his mom we forced him to move here, he has no clothes here (he has piles of name brand clothes) and he’s going to move back with her soon. I know it isn’t up to him, legally they are their dads now full time and it would require a court battle to switch it (their mom is TOO lazy for that, she signed the custody papers right away for them to move with us so she didn’t have to go to court). Their mom hasn’t brought any of this up to us besides the clothes thing, but I’m getting SO sick of hearing we forced him here when we didn’t, and he wants to go back. I’m not worried about her getting custody back, but I’m pissed I provide so much for this kid just for him to try and play both sides. We are much more well off than their mom and I think that’s the biggest reason he wanted to move. They’re not spoiled by any means either, so I’m just confused why he’s acting this way. Makes me not want to do anything for him!