This could be a deficiency usually niacin and could be he isn't getting enough even though your using brewers yeast. What dose are you using to start with?
If you don't see any out ward injury or redness or swelling it could still be an injury but it won't hurt to add some liquid B complex to his fav treat and see if it is a deficiency Here is a good link that shows what to get and how to give it.
And please let us know how he is doing. Oh and what are you feeding him? and be sure to keep him for over exerting while he is like this rest and water therapy and B Complex is best right now. [Make sure he can get in and out of the pool easily also]
https://www.backyardchickens.com/th...s-niacin-riboflavin-deficiency.1228833/page-3