Do you have a scale that you can weigh the duckling on? If not, can you get one? When they get sick like this it is a very good idea to start monitoring them for weight loss because if they lose 5-15% of their weight you will have to start tube feeding.
If it were my duckling I would switch feed to Purina Flock Raiser or Purina Duck pellets and I would give it some Rooster Booster Poultry Cell and some B complex for cattle.
I would start weighing daily, and if it was not drinking I would tube it some Gatorade or Pedialyte, wait 60-90 minutes and repeat. Once hydrated I would tube some duck food or Kaytee Exact baby bird food.