How puggy is the dog? If he is very flat faced it may be an ongoing problem.
We have a Frenchie, but she is not very flat faced, she has a decent snout and nostrils, so can breathe quite well, but we are still vigilant in hot weather.
She has a cool mat, but prefers to lie on the laminate flooring.
We also keep our bedroom curtains closed on very hot days to keep the sun out of the room, it helps to keep the temperature in our bedroom down and she often sleeps in there as it's cooler. We have a fan in there too and if I spot her in there I turn the fan on.
If she is up and about I will often drop a treat into her water bowl. She ends up drinking a lot of water trying to get the treat.
Never walk a flat face in the middle of the day in hot weather. I try to be out and back by 8.00 a.m. or going out around 8 p.m. and if I think it's still too hot on those really uncomfortable days, then she doesn't get a walk that day.
I haven't done this myself, but you could try laying a damp teatowel over him to bring his temperature down.
But if in any doubt - call the vet. Flat faces can go past the point of no return quite quickly.