Spray the horse's body with cool water.
Stand him in the shade or in a cool ventilated barn.
Let the horse have a few swallows of cool, clean, fresh water every few minutes, but DON'T let him have unlimited access to water or he may drink too much, too fast which can cause other problems.
In severe cases, place ice packs on his head and on the large blood vessels on the inside of his legs.
Call your veterinarian and describe your horse's symptoms. Your vet will know what to do.