They do get lethargic in the heat, so you could try some cooling techniques and then see if he behaves more normally. If not, then take him to the vet. In the meantime, keep an eye on his poos to see if they are coming out normally.
The info. page on this site has some tips, but my 2 favorites are: take an empty plastic bottle (coke, or seltzer etc.) and rinse it thoroughly, then fill with water. Put it in the freezer until it’s frozen. Lay it on its side (tightly screw the cap closed) in his house, maybe against one cage wall. He will lie against it and it will cool off his body. Also, as it melts, the condensation can be licked, so be sure the outside is clean and the label is removed.
My other fav. is to take a washcloth, wet it with cool water, wring it out so it’s almost dry, and wrap both ears in it like a turban. My bunnies used to let it stay like that on their heads for quite a while. The inside of the ears is particularly sensitive to being cooled with a cool cloth. Not freezing cold.
Remember, a bunny will doze during the bulk of the day anyway, so when it’s hot, it’s a good time to stay in the shade for a long siesta.