I agree with
@Quatie with switching to making a liquid with duck pellets in a blender as the Kaytee hand feeding formula is not designed for waterfowl who require much higher levels of vitamins and certain nutrients.
You said he won't fight for food when he is with the others. Do you have multiple feeding stations spread far enough apart to prevent bullying?
I know I sound like a salesman at this point (I promise I definitely do not get paid they're the ones taking all of my money

) but Rooster Booster's Poultry Cell is my go to. It has saved my ducks many times and has worked better for my flock than anything else. I've even seen results within hours with ducklings I've had flopping over using a good dose of it. However, if he is deficient in Niacin it could take a full week to weeks to see any further improvement. The more deficient the more you have to work to build back up the levels to the point where their bodies can maintain those levels as well.