it really doesn't matter what the reasons were- he should not have been assaulted.
Why is it so hard to understand that what you are doing is hurting the assaulter by allowing him to think it's ok? How does this help him?
Another note: you are saying the black kid couldn't be "expected" to show restraint, right? Is it because he's black?
Nope. Again to point at my son, I would demand that he show restraint. There are more effective, yet non-violent ways of dealing with mouth-breathers like that.