Sounds a bit neurological. Try giving him extra niacin: durvet high level vitamin B compound. We give it orally in ducks, 1ml soaked into a treat so that you can watch him eat it give it for a week. 8f there is improvement, continue for another week and add nutritional yeast or brewers yeast to the ducks feed: 1 tablespoons brewers ready percup of feed. All you ducks can eat the feed with added brewers yeast. Let us know how things develop.