Thank you for caring for this little one
You did right to give sugar water initially. Please, now, get oral rehydration solution from the pharmacy shelves at your supermarket. If you can get flavor and color free Pedialyte, that is best (I have found it at my local Walmart) but grape seems to me to have the least coloring and other additives.
The reabsorbed yolk sack will provide nutrition for 1-2 days. Get a small bag of non-medicated poultry starter crumbles and some nutritional yeast (i find that Walmart own brand is the cheapest, but any other brand is just as good.) To get the little one eating, dip your little finger in the oral rehydration solution and then into the crumbles and offer your finger tip to the duckling. You might have to put a crumbles just inside the bill if the duckling doesn't nibble. Keep offering crumbles on your finger, frequently, initially. Keep the little one at 90F and let it rest between offering crumbles. I have rescusitated very sick new ducklings in this way. The intensive feeding by finger tip usually pays of within 24 hours. Once the duckling is feeding itself, add 1 tablespoon nutritional yeast to every cup of starter crumbles. Ducks need more niacin than chicks, and sick ducks need even more. Best to use nutritional yeast to prevent niacin deficiency. Don't wait for symptoms.
The duckling needs to be kept at 90 F for 1 week, then 85F for the second week, 80 for the third week and 75 for the fourth week. I find that fourth week ducklings are OK at lower temperature while running around during the day. But need 75F when they are resting/sleeping at night
Do let us know how you get on and post photos. We are all gaga about little fluffies!