Aggressive behavior.

The Boy has been having a bout of aggression lately. Until we came back to Oregon I worried that he would be too passive. Other kids would bully him and hurt him because they knew he wouldn't do anything about it. (This happened several times with the cousins kids who were all within months of him in age. 1 boys and 2 girl) That's changed now. He pushes back, and he hits and he grabs other kids when they annoy him. (Sigh) I need to find a way to teach him that he doesn't have to just sit there while other kids hurt him, and that it's not okay to hurt other kids even if they hurt him first. He is normally such a sweet kid. Last night he cried when The Other Boy was in trouble. And yet, on Monday he was in trouble for pushing and shoving at school and today at school he hurt another child. According to The Boy this other child (a boy) hit him 2 times in the stomach before he grabbed him . He says he just wanted L (the other child) to stop hitting him. I understand, but he had such an extreme reaction! He's in time out right now because of it and won't get gummy slugs for his snack later. We talked a long time about why it's not okay to hurt someone else. He seems to understand, but does he really understand? What am I going to do if he keeps this up? He isn't aggressive with adults, maybe because they are bigger than he is or because adults don't often show aggression toward him.