Just going to throw this out there- since he is boarded, are you 100% sure he is actually getting those amounts?
I am a veteran on the feed industry- have done many, many farm calls over a 15 year career in the industry. I worked with a woman who could not get her horse to gain weight, even with being on 10 lbs. of Triple Crown Senior.
She had the vet out, ran tests, teeth checked, dewormed, you name it. Vet cannot figure out why this horse isn’t gaining.
She sells the horse, and he immediately starts gaining weight. It was then confirmed by a farm employee and another boarder that the horse was never fed the amounts recommended as the farm owners did not want to spend the money on feed.