Getting him fecal tested and dewormed if necessary would be the first step, along with getting his teeth floated. Rice bran is pretty high in fat, and I highly recommend it for putting weight on. You do have to wet it though. If there's not a lot of grass and hay available, I would feed a couple pounds of hay pellets or cubes a day to make sure he's getting all the forage he needs. If there's a suspicion of ulcers, aloe juice can help with that. A good pelleted feed (no sweet feed), or a senior feed if it's appropriate, will be good.