Don't worry about it too much. It's like that here, too (not too far from Michigan). Like stated above, make sure he has plenty of water. All horses sweat, even when they're just standing in a stall. If I were you, I'd let him into the pasture (if there is good shade) so he can stand in the breeze and fresh air.
My horses are outside in the pasture 24/7, where they have access to lots of shade and a creek so when it gets really hot they wade in the water & splash each other