It sounds like your kitty might be shedding in reaction to the heat wave, and in cleaning himself, is swallowing some extra hair. Have you ever tried the furball remedy? There are several brands, but they all look like brown goo and come in a toothpaste tube-like thing. Your vet should be able to recommend a good brand for you. Also, you might want to try brushing his coat out every day, to remove the loose fur that he is otherwise swallowing while grooming. The fact that it's hairballs coming up is a fairly good sign - hairballs are usually pretty easy to keep under control, once you see your kitty having some trouble with them!

Good luck!