Obviously he will still need some exorcise so do this in the early morning or evening when the tempreture has cooled down.
watching my dog, he always chooses to lay down on the tiles or floor-boards in hot weather.
But don't walk dogs in the evening if you will be walking on pavements for any part of it. The pavement is still hot and hurts their feet, so go first thing in the morning.
lankeela has a good idea. if you keep the water spray in the fridge until you go for a walk (or even when the dog is just roasting) and then spray it at the back of the neck it'll cool him/her down. and you can use it to spray yourself...
I had to laugh at the suggestion about the kiddies paddling pool, at this exact moment my daughter has the paddling pool on the lawn for my 2 year old grandson and both he and one golden retreiver are stretched out in it!
If you have to leave your dog alone indoors try and restrict him to one room preferably in the coolest part of the house and shut the blinds /curtains to keep the sun off. I have to leave my two for a few hours whilst I'm at work, I put them in the kitchen with all the blinds closed and plenty of chilled water. I find it's quite cool in there on my return and have started to shut the curtains on the sunny sides of other rooms to to keep the rest of us cool.