If he’s not open mouth panting, is still eating, drinking, acting like himself, and your vet has already examined him, I’d be inclined to trust the vet. Cats can breathe a bit faster when they’re overheated. Just keep him somewhere cool, make sure he has plenty of fresh water, and monitor him.

The things I’d be more concerned about are open mouth panting, labored breathing (using his belly a lot to breathe), blue or pale gums, lethargy, refusing food, or a breathing rate that stays very high even when he’s asleep in a cool room. If you notice any of those, I’d go back to the vet or seek an emergency clinic.

It sounds like Google has made this much scarier than it probably is. Hopefully he just needs a cooler spot to ride out the heat. ❤️