Low grain with the highest amount of calories.
Has he been checked for ulcers?
This is my weight gain remedy
1.check for ulcers
2.treat with omaprazole paste and equishure then switch to Animal Element to keep from re occuring
3.high fat low starch diet
4.Low NSC feed such as Ultium...Startegy HE...TC Senior..Powerglo with 1 lb of Nutrena Empower added for fat.
5.Hay 24/7 with 2 flakes of alfalfa per day.
I've never hadf a horse NOT gain weight from this regimen. Barrel horses to OTTBs to trail horses to rescue horses.