I don't know the specifics as far as the pony's age, weight, history, but if you have a muzzle it sounds to me like you're being careful to avoid any potential for foundering. Do you have a separate place, a paddock or other dry lot type area, where you can or do feed hay? If so, you can feed hay, and then let the pony out with the muzzle on if you're concerned about him overdoing it.
I don't know how much help this can be for you. Ideally I think a vet who is familiar with your pony, his specific living environment and his ideal diet will have the best advice.