Like we are trying to say niacin deficiency can hit only one in a flock or many, it's an individual thing and probably for some reason he just isn't getting the amount of niacin his body needs, Treating for niacin deficiency is usually the problem when one has weakness which would show up as...
Sure sounds like a niacin deficiency or possibly a toxin. But both can be treated. Niacin def with liquid B complex 1 ml over a tasty bit of treat just for him so you know he gets full dose. This will give you info on what to buy to treat...