I'll go ahead and suggest some specific feeds. What I would suggest first and foremost is a feed made for ducklings. However, in the US not a lot of feed stores carry such a thing. Purina just started making one, and my Tractor Supply carries it, but it only comes in five pound bags right now, and that's not really sustainable when you need to drive four hours out to the feed store each time. So it sounds like that's out.
Next up I would suggest anything that's called an all flock or Flock Raiser. Purina makes a feed called Flock Raiser that comes in 50 pound bags and is formulated to work for ducklings. This is a good choice. Naturewise also makes a feed that works well for ducklings. They call it Meat Bird and it comes in a bright orange 40 pound bag. The protein is 22%, but this is what I feed to all my ducklings until they are adults and it doesn't cause any problems that I have noticed. It too has enough niacin for ducklings.
Now, if none of that is available, then go for any unmedicated chick starter. This will not have enough niacin, so you will also need to buy something to supplement the niacin with. I would recommend buying the Vitamin B Complex I previously mentioned for this purpose.
If you can't find unmedicated chick starter, then get medicated chick starter. It won't hurt them, they just don't really need the amprolium so there's no reason to buy medicated unless you can't get anything else. Again, you will need a niacin supplement.
And since he is already having issues, I would suggest getting the Vitamin B Complex no matter what feed you go with, and giving him 1 ml directly every day. If he gets more vitamins than he needs, he will just excrete them.