Ducks, like chickens, need a maintenance feed of at least 15% protein. You can feed chicken feed, allflock or chick starter. A laying female will do fine on layer feed. I prefer to feed mine pellets, as they tend to try and choke themselves on mash, and have nearly as much trouble with crumbles. You'll probably have to supplement B-vitamins, as most chicken feed doesn't have enough (especially niacin) for them. Just buy vitamin B-complex (powder, liquid, or pills) and add it to his feed occasionally. Some people use brewer's yeast, but that's rather expensive, comparatively.