
When giving Niacin it isn't a quick fix it can take days weeks or months depending on how long the duck has had the problem.
I'm not familiar with Ad3e can you explain what this is?
Where do you keep your duck and what kind of bedding or ground is the duck on?
We do realize in other parts of the world people feed differently than we do and wheat is a good source of feed but at his age he does need the extra niacin. 10 mg per day.
here is info on feeding wheat.
https://happyfarmyard.com/can-ducks-eat-wheat/
I'd make sure wheat is his main source of diet with the other things you mentioned as treats.
people who have poultry in Iran feed this mainly as their chicken, ducks ,geese diet? or is there chicken feed too?