I have a few old horses myself. They get feed senior feed and barn mash plus a few other supplements to help with joints, coat and digestion. Horses age differently, many don't make it to 30 these days. His weight may be a sign that he is getting close to his time. But to help try feeding him barn mash, its easy to digest and easy on his teeth. Consider uping his feed a bit too, but not enough he colics. As horses get older its harder for them to graze. So instead of coastal or just free range, try feeding him alfalfa helps put on weight too. Also consider the feed, make sure its easy for him to simple lick up rather then having to chew it, easier on his teeth. Consider feeding supplements such as joint, and digestion, and protein to help him keep up his weight. You just baby the old ones cause you love them to death. They get the extra spoiled rotten treatment.
Patience is always rewarding in the end