Ideally, 65-70. Realistically, try and keep it below 80-85. I also give my rabbits granite tiles (12x12 floor tiles from home depot) that stay cooler than the rest of the floor and one of them has a small fan aimed at it so they can choose to be in the airflow of the fan as well.
You can also try offering him a frozen bottle of water to lean up against or sometimes I'll even put those granite tiles in the freezer and then give them back to them.
One of our members on here once recommended freezing leftover canned pumpkin since rabbits can't eat a whole can at a time. I freeze little portions in ice cube trays and then give them the still frozen cubes of pumpkin and they seem to enjoy that as well as a treat.
Ooo I really, really like the frozen pumpkin idea. In the past I've made ice cubes with mint leaves in them and left little bowls of these around the house for my Melv to cool off with. I also put fans on during very hot days, but protect the cords just to be safe. It can get pretty hot here and I've also stuck my hands in the freezer to get them cold and then rubbed Melvs ears, I'm not sure of that help cool him but he seemed to really enjoy it.