he needs to eat more than cracked corn. it doesn't not have enough protein for him... that might be why he's not growing his feathers right.
He needs to go back on a feed. He can go on a grower and/or a maintainence feed.
Typically a duck's diets ideally should be half feed, half scratch grains(and/or cracked corn), supplemented by fresh treats every now and then.
Are you sure he's a boy? what does his quack sound like like? if he's really a girl (can't tell by feathers since he isn't growing them right) he will need calcium supplements soon and can go on a layer feed.
I think it's best you find yourself another feed store, and if it's far, think about getting a big bag and an air tight food storage container (like the kind for dogs)