Could be walking on slick floors is hard on their legs. Are you supplementing with any form of niacin? Even duck feed can not have enough for some breeds and Pekins are one.
Nutritional yeast is a good way of giving the extra they need. Just sprinkle 1 Tablespoon over each cup of feed you give...
This info will explain everything that has been shared with you. I'd get him on liquid B complex since he is showing signs of niacin deficiency.
https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/