Well just how long did he tell you to feed just hay? I mean it won't kill your bunny but its not going to be enough in the long run, I'm assuming thats he's doing it because of the antibiotics, and is trying to avoid gut stasis. I'd recontact my vet and ask for a written diet with definite timelines, see if you can give him some sort of supplement like Nutrical. I don't want to say anything thats going to contradict your vet, but I myself would want a clear idea of how long the bun is supposed to exist on hay alone.
I free-feed my rabbits, feeders empty I fill it, particularly in the winter, they need the extra calories to keep warm. They always have hay, sometimes I topdress their feed with a little sweet feed and Animax, which is basicly the same as Calf Manna. They get some bread products as a treat, and live green forage in the growing months, and stuff from the garden. I will say, the minute that I suspect gut stasis I pull pellets and up the greens/hay.
Talk to your vet, he may have a good reason for the hay diet.
Calf Manna is a supplement sort of feed, 'Manna Pro' I think the name of their rabbit food. Calf manna can be fed to all sorts of livestock, but you have to be careful with rabbits, its very rich for them. Its also supposed to really push milk production up, but you have to stop feeding it when the babies are weaning because there have been instances of problems with mastitis that may have been linked to the use of Calf Manna. I haven't had that problem but have heard of it, figured I should put it out there. Often you can buy small bags of CM at rabbit shows from vendors, otherwise your stuck with a huge bag of it, I bought mine nearly 3 years ago and still have quite a bit left, sealed up in jugs. I only give each rabbit a few pellets at a time. Think of it as a vitamin, not a feed.