Your dog does not feel the heat as you do. His body is designed to conserve heat.
A dog’s sweat glands are located on the pads of his feet and nose, and they are inadequate for cooling your dog’s total body mass on hot days.
The only innate cooling mechanism your dog has is panting. If a dog breathes in hot air, especially if he is panting, it doesn’t take long for brain and organ damage to occur due to heat stroke.