I'm so sorry, I don't know how to help, I don't have any experience with this sort of thing. But I thought I'd go ahead and say if you can, post pictures. It may help whoever does know how to help.
Have you been giving him either food specifically for ducks, brewers yeast, nutritional yeast, or niacin in his water? niacin deficiency is the main thing I've heard of causing leg issues. I'm really not sure about the eating problem...
Is he pooping normally? Is he active or is he acting lethargic at all? Does he have access to water (deep enough for up to his nostrils?) whenever he eats?
There should be more people online soon, as it's just getting to be morning in the US and that seems to be where most people on here are from.
Good luck! I hope he's okay, and it's great of you to be doing what you can for him