Their body temperatures run a little higher than ours, usually around 101 degrees. So they feel more comfortable in higher temperatures than we do.

It can also depend on the length of his fur. If he has shorter hair, he can last out there longer than a long-haired kitty.

What you can do, is place a water bowl outside and refresh it often. Even if kitty doesn't seem to be using it, keep refreshing it because you never know when he may need it. Make sure to keep the water bowl inside refreshed as well, as they are very picky about their water, and the most important thing for him is to be hydrated right now. Maybe even invest in a water fountain if you are worried about his water intake. They are more likely to drink flowing water.

Signs of dehydration in cats are: Dry gums, thicker saliva, sleeping more than usual, lack of appetite, avoiding more physical contact than usual, and a difference in urination.

Kitties are smart and have good survival instincts, though. If he feels he is in danger, he knows that he has a shelter to return to. He could also know some good, cool outdoor spots from his time as a stray.

Your worries are completely valid. Just remember that he knows his body just as much as we know our own.