You have gotten great advice. I would strongly encourage you to get the book From Emotions to Advocacy by Pete Wright. It is very, very helpful.

"and yes, this simple incident at lunch time ruined the rest of his whole day== he wasn't able to get over it."

This would concern me. I think there are other parents here who would have better understanding and advice about this kind of situation--but I hope he can get some help because the odds are good that in a school full of kids, similar incidents are likely to happen with frequency. (I would have been annoyed too--but the not being able to get over it part makes me sad for him).