It seems to be a very common problem with this generation of kids.
ONe thing that at least kept my kids interested in even showing up at school
was that they both were in band or orchestra. Music was the only thing my older son was passing at one point in 6th grade.
You sound like YOU are doing the right thing. You are engaging him in things
that you do and he Used to like doing.
My younger son went from being a happy silly 11 year old to acting like a ghoul when he was 13, 14, 15... He hardly moved his mouth when he talked, he kept his hands at his sides and when he walked around like that it was bizarre. The transformation was awful for us, we didn't know what to do with him (and of course he couldn't bear to be seen with us in public)
It was all so scary for us, it really was.