Do you mean that he is only getting 1 flake of hay/day? If so, I would say that he needs a lot more hay/day.
What kind of condition is he in? I realize he's a rescue and he came off the feed lot and if a horse is severely emaciated you need to gradually introduce more to eat so as not to overwhelm their system.
How long have you had him for now? What is his body condition like at the moment? Have you had the vet out to see him? If so, what did he say?
The most important part of a horse's daily diet is hay and/or pasture, then build upon that. You could try a ration balancer. Ration balancers provide a lot of nutrition with only a small amount having to be fed/day. They are a good way to supplement free choice hay/pasture and give the horse all of the vitamins and minerals that may be lacking in their hay/pasture. Deworming your horse is also important to be sure he will be getting everything he needs from what he is being fed.
I really think you should ask your vet for advice though if he is in really bad shape, cause you don't want to overwhelm him. Also, if he has never been dewormed before and is full of worms, it would be best to get your vets opinion on what dewormer would be best to start with.