To give electrolytes is a good trick. You can put it (in powder) in the food or give it to the horse with a syringe (in his mouth, as a vermifuge). It contains vitamins and many ingredients helping the horse to recover and get back the elements lost when he sweat.
I have already faced this problem. So I have 2 solutions. First I put the hay in the water, so the horse has to drink a little bit if he wants to eat the hay, the hay is wet and gaves a better taste to water! If it doesn't work, I put a salt block in the box (my horse loves it, so it may not work with every horse), which makes him thirsty. He ends up drinking!