Can his foot be placed into the right position? If so, I can make him a little boot that would go up most of his leg (To the knee) to hold it in place. He might wobble a bit, and may have a hard time getting around, but it'll hold his foot in the right position while he grows and might help over all. How is his other foot? Is it okay looking? Are his legs both in the right position (No splaying?)? For babies, I'd have em wear two shoes, one on each foot so they don't trip or stumble too much. Babies are not always fond of wearing shoes and will sometimes not walk. It takes them awhile to get used to it.
I am unfamiliar with muscovies, do they grow at the same rate as a regular mallard derived breed? Meaning, at 4 weeks old are muscovies about the same size as a 4 week old pekin? Or do they grow faster? Your little guy is probably going to need two different baby sizes and then one or two adult sizes (depending on if he turns out to be a he lol) to help hold his foot in place. I've been working on shoes for other ducks that have various deformations (boots that need boning to stabilize them), so this'll be another experiment for me. If you PM me an address you'd like me to send some test shoes to, I'll be more than happy to try to make something for him no charge. Just keep me updated on how it works so i know if it'll help other ducks in the future. That and if you take pictures, let me know where I can see them! I'll get to working on his stuff tomorrow, since getting his foot in the right position asap is what's best for him!