I'm not sure what else you can do other than possibly adding a super B complex vitamin to the niacin. B12 gives energy. I just am stumped. Have you examined his legs to make sure there is not any swelling or redness which would mean an inflammation or infection. and how is he other wise still eating good not looking depressed.In the water he can move fine without any trouble but once I take him out of the water he just sits down and tries to walk and I have been giving him 500 niacin everyday in his drinking water
I have a drake going on 10yrs old, He can barely walk but is able to get around some, but I have to carry him to his pen of a morning and back in the evening we treat him as elderly and will give him all the time he wants as long as he is eating still and likes to still have interaction with his flock mates who are almost as old as he is. Once he gives up then we'll have to say good bye.
I am saying this to you because if he is acting normal and your willing to give the commitment maybe keep trying and see if he can have a quality of life. Not everyone can do this though it's basically a decision only we can make.