My horse is 45 years old and is just now losing a lot of weight.?

I feed him 2 quarts of sweet feed and 1 quart of alfalfa pellets in the morning and the same at night. He does eat all of it. He nibbles on hay but does not have a whole lot of teeth left to chew the hay. I'm looking for an inexpensive way to at least put a little weight on him. Otherwise, he grazes, eats gr and seems okay.