Our 28 year old pony had a bad winter and needed to be stalled and penned seperately from other horses so she wouldn't be bullied and could eat all day(hay).
We put her on corn oil, Vitamin E, Senior feed, Equishine, Rice Bran, and weight gain.
I highly suggest making an exact list of what you feed him and submitting it to your vet. Most vets can measure how much you are feeding the horse by their weight and can tell if they will gain weight from that food or if you need to feed more. I hope this helps!