Sorry to hear your duckling is feeling poorly.
We are limited to the amount of treatment we can provide to a small duckling due to their size, and fragility; but the main things you will want to focus on is ensuring the duckling is being kept warm and in a stress-free environment.
If the duckling is willing to eat and drink you can try dripping a poultry vitamin alongside its bill, being careful not to aspirate the chick.
You can also try offering some mashed scrambled eggs for some protein several times a day, or offer soupy warm feed.
A lot of the time with these young ones we are dealing with fluid, nutritional, and electroltye deficits, so this would help address all.
Hopefully, you will see some improvement soon.
https://www.tractorsupply.com/tsc/product/nutri-drench-poultry-4-oz