When one is having issues with lameness it's best to get liquid B complex and start them on it asap. Even if feeding the best duck food out there one can still come down with Niacin deficiency.
I'd change over to Nutritional yeast Or since he is having a problem get Liquid B Complex it actually works better when one has already a problem.
1 ml over a small tasty treat so your sure he get full dose . TSC usually has it and it says injectable but we use it orally. it never hurts to try...