Maybe put him on a higher protein feed? And add more portions + veggie oil? That's what wee did with our horse (Tornado), he was 300Ibs underweight when we had the vet check him. Now I would say he's anywhere from 100-75Ibs underweight after a year of TripleCrownSenior (at 4 cups a day) + veggie oil. He was ona 24/7 roundbale of fescue..