It looks like a balance issue , like he’s just trying to get his balance. I do have an eight-year-old pekin female who uses walls or fences to press against while she tries to preen, Because her legs are pretty tired of holding her heavy little body up at this point, and she starts to fall over when she tries to reach her tummy for preening.
When you say that he has been on niacin supplementation all his life what do you mean what product? What form of niacin and what amount?
Also, if you could get a picture of his poop and post it , that can help figure out what’s maybe going on in case there’s something infectious or neurological.
You can also Check both of his legs carefully feel them up and down all the way from the toes up to the hip joint compare them with each other. Are there any warm spots? Is there anything that feels bulging or out of place?