Is there some kind of deficiency in his diet that causes him to want the protein in feathers?
No. He is simply taking advantage of free protein floating around. That is just something chickens do. It does not signal a deficiency. It is not something weird. If he were plucking feathers off of the others it would be a problem but it doesn't sound like that is the case.
I guess I just assumed that you couldn't since I don't feed scratch until laying age.
A broody hen feeds her chicks stuff we would consider scratch or treats soon after she brings them off the nest. First she takes them to a spot where they can get grit from the ground. You may not feed scratch until laying age but a broody hen sure would.