The best thing to do if he reacts childishly is to ignore him. I know it's not the easiest thing (anger makes it hard not to focus on whatever caused it in the first place, after all), but unless you can trust yourself not to rage, it's best to try and hold back if you can.
I don't know if you're doing these things already, but honestly, if it's getting to you, it helps to either vent to a friend in private or even just type out the stuff that makes you rage out on a word doc or a blank pm or whatever equivalent you prefer.
...If I sound patronizing at all, my apologies in advance, I'm trying to be helpful, not annoying. ^_^"