How long has been since he's had a physical, including some blood work and getting his teeth checked?
It sounds like maybe the vet was involved when he got hurt last November BUT did your mom's horse get a physical, at that time?
If not, I would start there while, removing the corn and sweet feed at the same time. For all the reasons others have stated
Beet pulp is good to add weight but even the pellets should be soaked.
Very well soaked timothy/alfalfa cubes should not hurt him but, if he has metabolic issues, even at his senior age, the alfalfa may raise his insulin.
Soak them down into grass mush.
It would be great if he can tolerate a small amount of alfalfa cubes as it is good for soothing the stomach.
If he doesn't have ulcers by this juncture of his life, he is doing fantastic -- he is doing fantastic anyway - smoochies on the snoot of his sweet face