How are his teeth looking? Does he have any?
He could possibly be anemic. You can either have your vet do some blood work, or start him off on Red Cell and see if you notice any changes.
My sister's old Anglo Arab is here now. He has very little teeth
So he gets Purina senior, alfalfa pellets, ground flax seed, arthritis supplements, red cell, and daily dewormer- whew! All mixed up with hot water- made plenty sloppy
TJ has been putting on quite a bit of weight. I know the redcell has helped alot, as TJ's anemic. Before it didn't matter what he was fed, he just wouldn't gain. Now I can see a hint of ribs, but I sure can't count them! He still has a ways to go. He's always been a hard keeper, but now that he doesn't have much in the way of teeth, it makes it even harder