I would suggest either a dark plum-ish color, or black. You can't wear anything with brown, it wouldn't compliment him very well. While black would no doubt be hot, you could combat the heat by getting a
linen dress from B.Rep that comes in black and doesn't cost too much. I think J.Crew may have a few linen dresses currently too. That's my opinion...nothing is better in the summer then linen!