One of the main reason your horse has a heat stroke is because he's in dehydration. So you should rehydrate him asap. Rapid rehydration will not be possible with water alone as electrolytes, particularly sodium, are needed. There are a variety of commercially available equine oral replacement electrolyte solutions on the market, but it is important they are made up at the correct concentration and that your horse will take them. Adding apple squash or apple juice helps to mask the taste
First of all, you have to call a vet. While waiting your vet, you should:
Keep your horse cool by doing a nice sponge bath, or even rinse him off with the hose to cool things off a little. Try to hydrate him, and keep him out of the sun if possible too.