Your dog's normal temperature is 101 to 102.5 degrees Fahrenheit, so any temperature over 104 degrees means he’s in trouble and requires attention right now. If, because you haven’t followed some common sense precautions, it gets to 106 degrees, this constitutes a dire emergency and damage at the cellular level begins, as well as potentially irreversible damage to his brain, GI tract and kidneys. Additionally, at this temperature, heatstroke is imminent. If your dog's temperature gets to 109 degrees, he will have a heatstroke, period. Some of the things you might witness if your dog is in heat distress are: dizziness or disorientation, vigorous panting, dark red, tacky gums, thick saliva, lying down and unwilling (or unable) to get up, collapse and loss of consciousness. Once the signs of a heatstroke are detected, there is precious little time before serious damage or death can occur. 

First, move your dog out of the heat right now. Shade will work, but air conditioning or a fan is better. Begin lowering his temperature with cool water. You can put wet rags or washcloths on his foot pads and around his head, but replace them frequently as they warm up. Avoid covering him with wet towels, because that will actually trap heat. Offer him cool water, but don't force water into his mouth. And don't let your dog drink excessive amounts at one time. Call the vet and let them know you're heading their way. Do not use ice or ice water. This can actually cause his internal temperature to rise further. When his body temperature reaches 103.9 degrees, stop cooling him. At this point, his body should continue cooling on its own.