Best one that’s 1 ml daily. You can serve it over something they love maybe peas or mealworms, just enough treat that the duckling eats the whole thing so he gets the whole dose. Water melon is another fav here.
I would get the liquid B Complex. If you have a TSC pretty close you can pick up one in the cattle dept it will say injectable but can be given orally. Or you can get human liquid b complex. And most pharmacies carry it Walmart too. You need to start your duckling ASAP or it may never walk...