Be really careful with heat exhaustion--- dogs can go downhill fast. I had a scare the other night and learned a whole lot more than I wanted to about it. I waited til it was almost 8:30 at night to walk the dogs and it had cooled considerably. Two of my dogs were fine on a short walk but by the end, one of my dogs was breathing heavily and salivating. It scared the heck out of me and did some research.
I got him to stand on a cool, wet towel. I also put a cool, wet towel around his neck and head. He finally cooled down but it was very scary and I learned that you have to be very careful because it can sause permanent organ damage and death if their temperature rises too much. (You also don't want to cool them too quickly because that can cause problems, too).
If he is having problems, I would keep the walks very short until it cools down.
Holly, Oliver, Rosalita, and Scarlett