Have you considered group therapy with your son as a means of opening lines of communication? It sounds like he's becoming recessive, and speaking from my experiences doing the same thing it's a dark place to be that ever 10 years later I'm still trying to crawl out of. I'm not suggesting medicating him, but perhaps a professional that could help you and your son talk to each other would add a bit of a buffer and make you both more comfortable before the situation escalates into something more severe.
If your son is anything like I was at that age, and from your description it sounds like he is, then letting his father go off on him is most likely going to cause some serious harm, it certainly did with me. I don't know if you've got the option to put his dad in check, but if you could make sure that he remains calm when dealing with it that could also help your so communicate, I was so scared of my dad that I never said anything because it always ended up just making him more angry.
My last comment is YOU GO BOY!! I think the little brat falsely accusing him should've gotten kicked, but don't tell him that I said so
I think you're right with encouraging nonviolent resolutions, and I think it might be worth mentioning to him when you talk to him that while responding to such things is necessary using his mind to embarass the other boy is a lot more lasting and effective. Aren't I terrible?