The best I can do is wish you luck. I don't know if daubing the umbilicus with mild iodine would help or not...we do it with foals, but I just don't know about how that would affect a duckling, or if it would be too strong for him. I was present at birth for my foals, so I got them daubed double-quick to dry out the umbilicus and prevent infection from entering. We don't have to worry about anyone getting yolk with those though...when the mare stands, you try to delay the breaking of the umbilicus as long as possible so any blood in it, which can be considerable, gets dumped into the foal and once that cord breaks, that's pretty much it.
Good duckluck to you!
Good duckluck to you!