when I read this it sounds just like what I went thru.
Is your son the middle child?
my son was called names things throwed at him and got in trouble for things he didn't even do. so after so long of it he started doing things. he even went as far as going to the teachers about what other are doing to him they just ignored it. so he started taking things in his own hands and he got in trouble. I did everything I could but nothing worked. later on as the years went by I realized he was being bullied and I didn't think the school allowed that kind of behavior until one day I got a call from the school and had to go to court and the other kid admitted he was bullying my son and others.
as I have learned through the years I should have believed him. Have you sit down and had a serious son /mother talk with him?