Something similar happened to my brother when we were kids. A kid was coming to school and testing out his new karate moves on my brother -.-;
I remember dad taking him into his room one night and trying to teach my brother how to box! I think mum ended up speaking with the other boys mum, and his parents & the school explained to him that his karate was for self defence and not fun...
My brother is a very placid kid, and so incredibly laid back I sometimes wonder if he's even aware of the world around him XD
I think you should be proud for one. You have raised a son that doesn't listen to a bully and knows what the right thing to do is, and he is only 8! It's great the school have aknowledge the situation and are taking resposibilties in making sure it doesn't happen again.