How about you edit the title to say something like Rehabbing wood duckling first time need answers?
If it is poisoning, then a slurry of activated charcoal - a food grade supplement - may help. If metal, something to chelate it, cannot recall the material recommended.
A rehabber could help you the most, I feel. They know how to raise and prepare ducklings for return to nature.
Meanwhile, I would drop back and get a wide-spectrum, complete feed from the feed store. Mazuri makes waterfowl food. And I would get poultry vitamins-electrolytes-probiotics to...