It's been really hot and humid where I live, so I have been very cautious with how much exercise I give my horses.
I will ride for 30-45 minutes when it's this hot out. This usually includes mostly walk/trot and a few laps of canter. I never jump when it's this hot out, although I will do poles if they are set up.
After I ride, I untack, hose him off, wait for him to dry, and if he's still hot I hose him again. Then I put him in his stall where he has a fan blowing on him.
I have noticed that he doesn't drink a lot... I use automatic waters. I'm not sure how much he drinks in the field either. Is there any way to give him a little extra water?
He has a salt lick in his stall and he loves it and licks it all the time, but then he doesn't drink afterwards. He gets some garlic in his feed too.