pop some bricks in the freezer, and then put in his cage when they're nice and cold. Or freeze water bottles of water, and wrap in cloth or something...frozen cranberries instead of dried as a treat, frozen banana might work, too, but I've never tried that. If he's an outdoor rabbit, make sure he has plenty of good shade, and on really hot days consider bringing him inside. If you have a small fan, you could put it on an extension cord, and have it facing his cage.
I suppose you could also try refrigerating his hay and pellets for say, ten minutes? lol. Just so he has cool food. Maybe that's overkill, I don't know; how hot does it get where you are?