If a horse sweats very easily, could there be something wrong?

My horse sweats a lot very easily when he is being hidden. He is a gelding Tennessee Walking Horse, born in 1995 and is 15.3 hands high. He's also hyper active and and sometimes breathes heavy if he has a lot of energy. He do saddle seat, english, hunt seat, trail, and now western. We got him in April 2006 and rethought (or retrain) him everything from ground manners all the way to what he knows now. We use to think he just works too hard on himself during the most easiest little thing, but we fixed that problem and he still sweats a lot and very easily. We live in more in the southeastern part of Michigan, and we never ride when its hotter than 90 degrees or colder than 20 degrees. Training is usually about 45 minutes long, and sometimes less and ending it on a good note. There's also plenty of walking breaks as well. Is there a way to solve, fix, or at least help control his heavy sweating problem?