Rice bran and oats ?

Well, I ume he is getting good quality hay or gr as well? I hope the rice bran is mineral balanced for horses, or he will be getting way too much phosphorus. Rice bran is a good fat source, and is fine as long as it is mineral balanced, but that is an awful lot of it every day. You'd be better off to feed a digestible fiber source like beet pulp rather than oats, and limit the rice bran to about a cup a day. But that is uming you are also feeding good hay. As long as your horse gets adequate protein along with other nutrients in his diet, he can build muscle by exercising every day. Be sure he has access to salt and mineral blocks, ample fresh water as well. For weight gain, I prefer Purina Equine Senior feed which is balanced nutrition and well digested. Fed along with hay, it is all you need to feed.