If he's deficient in vitamin A, he's likely also deficient in vitamin E (which degrades even more rapidly in hay). You might consider adding a ration balancer or vitamin supplement that is high in A & E to his diet. I'd look for >30,000 IU of vitamin A and >1,000 IU of vitamin E as a minimum in the daily ration. You can get that with 1 lb/day of Triple Crown 30% Supplement or 2 lbs/day of Purina Enrich Plus, for example. I've seen very few vitamin/mineral supplements that provide enough A & E, and none that cost less than a ration balancer.
If your horse is already showing signs of deficiency then it may be a good idea to give a high dose at first (like the injectable cattle stuff given orally) and then make sure he stays on a diet that's providing enough long-term.