I would get liquid B complex and poultry nutri-drench or rooster booster poultry cell as well as a few 3ml syringes (without the needles) to administer the vitamins orally if you can’t get the duckling to drink water with the vitamins.
These you can find at tractor supply.
I’m not sure what feed you have them on but it should be something like Purina flock raiser crumbles or Purina duck pellets.
If it’s a bacterial infection a broad spectrum antibiotic like baytril or amoxicillin would be helpful, generally those have to be prescribed by a vet “at least in my state that’s the case” or ordered as fish antibiotics.
If it’s a viral infection there’s not much you can do besides keeping vitamin intake up, keeping the duckling comfortable, and hoping for the best.