You've done well for him!
Don't worry about changing the feed-
You could try offering him treats like mealworms, though, that can contain a good amount of protein to give him a little boost. My ducks ate some of their feathers during a big moult, and I gave them mealworms, peas etc as a treat...
I'm just thinking out loud here, but possibly boredom?- do you see him eating the feathers? He could need more protein if so I think.
Is there a reason that he's an only duck? Do you plan on getting him a few hens/or a drake friend?
Ducks really shouldn't be alone