Unless the horse is over 15, the chances of it being Cushings related is pretty small.
You need to review your horse's diet, ensure that there is a free choice salt/mineral available, and ensure that there is plenty of fresh water availabe. Then talk to your vet about other possible causes of anhydrosis.
For general management, they should be provided ample shade, stalled with fans during the hottest part of the day and hose him down repeatedly during hot days. These horses should not be worked when it is hot because they can easily suffer heat exhaustion and heat stroke.
Licensed Veterinary Technician