Riders who don't keep their horses in condition for work, and who then take their horses out and use them, run the risk of serious problems. There are things which the horseman can do to help his horse make it through summer activities without succumbing to the heat. Always encourage water consumption. Try not to let extreme thirst develop in the first place. Give the horse frequent opportunities to drink.
Electrolytes are essential for the horse working in the heat.
Commercial supplements are available, but simple, cheap, home-mixed supplements will work, too. Two 11 oz boxes of Morton's Lite Salt. Bear in mind, though, that when his needs increase due to hard work in our summer heat, his tongue would probably become sore before he could consume all the salt he needs by licking a salt brick. That is why it is helpful to have it in the loose form so that you can add some to his feed. If you think you horse has special needs, consult your veterinarian.