It would help to know what and how much he’s being fed right now.
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.