What to do if your dog is suffering from heatstroke:

For the best chance of survival, dogs suffering from heatstroke urgently need to have their body temperature lowered gradually.

Immediately move your dog to a cooler area, either indoors where there is air conditioning or in the shade under a fan.
Use a rectal thermometer to check his temperature. Heat exhaustion typically occurs when a dog's temperature falls between 103 and 106 degrees. A temperature above 106 places him at risk for heat stroke. If he's in the danger zone, call your veterinarian.
If you're near a body of fresh water, such as a lake or a baby pool, let your dog take a dip to cool down. Otherwise, you can use cool, wet cloths or towels to help him out. Place your cool wet cloths on his neck, armpits, and between his hind legs, and you can also gently wet his ears and paw pads with cool water.
If he's conscious and willing to drink, give him cool, fresh water. Don't force it, however, as it may end up in his lungs. If he can't or won't drink, or can't keep water down, wet his tongue with water instead. Don't feed him ice cubes, which could cause his temperature to drop too quickly, leading to shock.

Once the dog is cool, take him/her to the nearest vet as a matter of urgency.