I would do shorter rides with your horse towards the end of the day.If you end up working them hard then make sure to give them a bath.Instead of riding you could do lunging excersizes, like doing cavaletti's on a circle or getting them to be responsive with noises like clucking,kissing, or woahing.Hopefully this helped!
The first thing that I would advise you to do is to ride early one the morning when the air is still a little fresh or if you can't, late in the evening.
Then I would do a lighter work than usual with more breaks between two exercises for example. If you have the possibility to ride out in the wood, it can also be a good compromise to find some freshness.
At the end of the session, bathe your horse and let him dry where the isn't any draft. Give him water and if the weather is really hot and your horse sweat a lot, you can add some electrolytes.