I model with my son a lot. For instance, when he explains he lashed out at another kid, I will tell him that I'm mad at what he did and disappointed that he didn't make a better choice, but I'm not hitting him, I'm talking to him about it. Then he'll usually start saying "calm down, calm down," which is what the school tells him when he gets steamed.
It's very possible that your son knows all about the other child's issues. My son tends to hang around with other ADHD kids, even though he knows the relationship could get explosive, because they understand each other better and are usually more accepting when a kid has a set back.
What I'd suggest is helping your son learn to recognize the signs that the other kid is about to blow his top. If he can realize when he should get away from him, then they can still be friends most of the time. I know - sounds good. I'll let you know how I get that idea through to my own son if I ever do...
I don't know if I can offer you any advice, but I can sure sympathize with your problem. Except, that my son is the annoyer, and the other child (who has sensory processing disorder and has frequent 'meltdowns' in class) is the one who physically assaulted my son. Both his father and I told our son that he deserved what he got after pushing this other kid's buttons, when the other kid told him if he didn't stop he would hit him.
The best you may be able to do is have the school keep these kids separated as much as possible; or, who knows, by the end of the year they may actually wind up to be friends. That's also happened with my son more than once - kids he could not get along with ended up being his best friends in the end.
Good luck, whichever way it goes!