I agree with miss Lydia that that looks like a niacin deficiency. What I use to treat it with mine is durvet injectable cattle b complex. It can be given orally. I like to give live mealworms as treats to wild type ducklings like that as babies to make sure they're eating well, so to get the b complex in them that's what I use. I use a small syringe with a needle attached, draw out some of the complex, and infuse mealworms with it. This way you can get enough into them and they don't even know. Give him a decent amount a few times a day and see if it helps. If it's a b vitamin deficiency he should fully recover in a few days.
And you can't really overdose b vitamins since any excess is excreted in the feces.
How do his hocks/legs look? Any heat or swelling?