My guy is on grass plus a round bale in his paddock with his face stuffed infront of it. And he is getting 3lbs of purina ultium and 2lbs of purina senior per feeding, twice a day, and he is also getting Ultimate Finish 100 - and he's still ribby.
I would have him scoped for ulcers, first thing. And I would look into digestion aids.
I have my guy on SmartDigest Ultra, SmartGut, SmartB1 and SmartC through SmartPak.
All suggested by my vet, and the digestive suppliments have been helping to ensure that his digestive tract is working properly, plus he is getting pro and prebiodics to ensure that he is absorbing what he needs to be absorbing to help him gain weight.
Ulcers will really impede a horse from gaining weight, so you might want to look into that for sure.
I'll look into alternate Ulcer treatment, but the TractGard seems to be working well. He hasn't given any indications of tummy problems since we put him on it.