Some people cool down the roofs of the cages by running a sprinkler system onto the roofs of the cages. In the mornings, the frozen bottles are placed in with the rabbits. The rabbits can lean up against the bottles to cool down. As the bottles thaw, they cool the surrounding air, making the bunny more comfortable. Wet Burlap or Towel.
Some people place a cool, wet towel or piece of burlap in the cage for the rabbit to lay on.
This also helps keep him cool.