[gottatrot]

For an old horse, the first reason they are thin is usually their teeth. Make sure he has his teeth checked by the vet to see if they need to be floated. He also may need to be wormed.
Since old horses often don't have good chewing surfaces, they may need soaked pellets or chopped hay added to their diet to help them get enough digestible calories into their system.

 

[gottatrot]

Soaked hay pellets or alfalfa pellets are great. We have Standlee chopped hay here that comes in bags, and it works well for old horses to supplement their regular hay.

 

[gottatrot]

If he's not getting nutrition from the hay, then replacing it with pellets means you have to feed a lot of pellets. That can be difficult, usually meaning several feedings a day with pellets that weigh 5 lbs or so each time before soaking. It is the same with senior feed, but hay pellets are much cheaper to feed in large quantities.