Racehorse people usually don't feed much hay, because they think a full belly slows the horse down when running, and a racing horse needs way more calories than hay alone can provide. So they give lots of grains. Sometimes the horses in training/ racing are also too tired to eat enough hay.
A horse that is not in this kind of work, can very well live on hay and/ or grass alone..... they did it for a long time before us humans became involved, and all wild horses still do. The digestive system is designed to eat constantly, high fiber roughage, small stomach, large intestines, and high production of stomach acids to start suggestion of the fibrous stuff. When the stomach is empty, the acid begin easing the stomach lining, creating ulcers. This could be what had happened to your house when he wasn't fed enough when he was gone from the farm. A vet can rest for that, and give you medicine for him, but if the original problem is not solved, he will just get new ulcers(a vet high% of race horses has ulcers, I think the number was around 90%). Then, to top it off, high grain rations can cause hind gut ulcers/ acidosis, which causes diarrhea and very smelly poop. You can imagine that a horse with these problems doesn't feel very well.
Once you give us a list of what you can get, and photos of his hooves, we can give you some ideas.