My biggest suggestion would be to keep him on a diet of roughage, as long as his teeth will allow, and lots of it. Have is teeth checked regularly, as they usually start loosing teeth around age 24, so my vet has told me. If he can not eat regular forage, like hay and grass because of his teeth, you can replace it with beet pulp or hay pellets or cubes moistened. There are several feeds that are formulated for older horses, but I can only personally recommend one, and that is Purina's Equine Senior. It is formulated to easily break down into a mash by adding water. It works best with warm or hot water though.
My greatest piece of advice would be to work closely with your vet to ensure him the best care that you can manage for the rest of his life. At 20 years young, he's earned that at least.