More than likely, you are dealing with a vit B deficiency. If you are feeding medicated chick feed STOP! It has a thiamin inhibitor and is making things worse. Mix brewer yeast in his feed. A crushed up human multivitamin will buy you some time to pick up something in the morning if nothing else.
Please post on the "duck threads" STAT. Ducks often have niacin deficiencies but, I don't know what medicated starter may have done to them. I don't know the amount of Brewer's yeast they need.
They were not fed medicated feed.
Dosage of brewers yeast is 25mg per pound of feed or about 1 tbsp per cup. If you put niacin in the water it's 100mg per gallon.
You can get everything you need at any drug or grocery store.