Sad to read about Waddles. I would not give up on him yet. Nerve damage takes some time to heal, and sometimes it doesn't.
Keep up with the extra niacin and if you haven't started multi vitamins (poultry vitamins), you might try that. His increased feistiness is a good sign.
How is his weight? Sometimes ducks develop something like arthritis, and extra weight is painful. Letting him swim in lukewarm water may help him keep some strength and range of motion without being very painful to him.
Some people have used a sling setup, similar to the little swings we put human babies in to let them strengthen their legs before they can stand on their own. You might try that while he's in recovery.
Causes could include physical injury, infection, virus, nutritional deficiencies (and by the way - some ducks are just more sensitive to diet so you have not failed him if that's what the problem is - it is impossible to know ahead of time who needs extra something). Keep feeling around the upper parts of his legs and belly.
I don't want to scare you, but he might have a tumor pressing on a nerve. If that's the case, I think it will become evident before too long.
Has he been mating? Fighting?
Keep looking for signs of something in his foot - thorn, tiny sliver of metal. Do his legs seem to have any strength when you're holding him? I'm wondering if he's finding it too painful to walk or if he just doesn't have the strength or coordination.