Dealing with a bully

My son is in 4th grade. Since the start of the school year he has been the victim of constant bullying on his bus by an 8th grader. I have spoke with the principal a few times about this.. The bus driver as well. Nothing has been resolved.. Things have continued to get worse. Yesterday, my son could not take it anymore and he hit this older boy in the stomach. This older boy then punched my son in the head. My husband feels that our son did the right thing and that he was only standing up for himself. I a little am torn about it... My son now is possibly facing bus suspension and in school suspension. I do not condone violence but how much can a child take? I am expecting a meeting with the principal to take place very soon and I'm preparing myself.. Do I stand by my son and his actions?