Talk to him about it. Obviously, be nice, and pussy-foot around the hard issues if you have to. Eventually, whether it takes multiple conversations over a period of months, or a couple of converstations within a week, he will hopefully alter his behaviour. He may not, but talking things out normally works for me, at least with people I know well or am close to.
Method number two is harsher, but still can work (can also backfire something nasty). You can just tell him he's being a dumbass or that he's pissing you off or whatever when the issue arises. I don't know that it's a good idea in this case, since he has suffered brain damage, but I've experienced it, and it can work as well. It ends up with a lot of conflict though, and is volatile in nature.
One final way of dealing with it, is to do just that: deal with it. Sometimes I feel a bit cliche bringing it into every facet of conversation I seem to have, but becoming a pseudo-Buddhist (I drink, watch TV, and a few other things that preclude me from being a "true" Buddhist in most schools) really helped me. Just meditating, finding a way to be emotionally, if not spiritually at peace (even deep breathing techniques can help immensly) could better equip you to deal with him. If not that, cathartic activity later, like boxing (doesn't have to be on a person
), exercising, sports, making voodoo dolls, whatever, could help you release some of this tension. Tension... oh yeah, you could beat off a lot too.