KY, has that horse had his teeth floated? Sharp edges on the teeth cause discomfort and sometimes ulcers in the mouth. Worth a try. If you can, run your fingers alongside his teeth to see if you can feel any sharp points or sores.
We have the kids old saddle horses floated every year or two, and it sure makes a difference in their ability to keep flesh on.
For what it's worth, I usually check the teeth on younger horses as well if they aren't dong as well as I'd like or exhibiting signs of discomfort when bridled, as they can sometimes have a tooth "hang-up" when they're changing teeth, or sharp edges on their molars as well.
Take care.