White poop just means that it is urates not food waste. Urates are produced by the kidneys and excreted into the poop as ducks don't pee.
I don't know what is going on with your duckling, but the best management is supportive care. I agree to the suggestion that you substitute oral rehydration solution for drinking water. Buy Pedialyte at your local pharmacy or supermarket pharmacy shelves. Best flavorless solution in bottles (i can only get that at my local Walmart) but grape flavor isn't full of nasty red coloring like the other flavors. Alternatively, buy sachets of the salts and make up your own solution.
Keep you boy somewhere cool, out of the sun. And quiet. I bring sick ducks into the house and keep them in a dog crate or pet carrier depending on how sick they are. Cover three sides and the top with an old sheet or similar fabric to reduce drafts and noise. Tempt the duckling with defrosted frozen peas, or finely chopped watermelon, or mealworms. Offer just a few every hour or so but have their regular crumbles/pellets available all the time. I add nutritional yeast to the crumbles (1 tablespoon for every cup of feed) as ducklings need a lot of Vit B3 niacin when they are growing, and more when they are ill. You can buy that at your supermarket (Walmart's own brand is cheapest) or health food store.
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