@thinkyesi , sorry to hear you the duck is feeling ill.
How long has he been reluctant to eat or drink?
Do you have any pictures of the duck's droppings?
Do you have a video of the duck?
Has the duck been around anything toxic (carcass, moldy feed, maggots, lead, metal, decaying vegetation,etc)
Oftentimes dehydration, and/or malnourishment will cause the bird to pass rather than the affecting condition - my first goals would be to address the bird's hydration, and nutritional levels, and then attempt to treat what might be the underlying problem. Unfortunately, determining the underlying problem at home can be difficult, to near impossible so sometimes treatment is founded on a conjecture basis which isn't most reliable.
If the bird is outside, could you place him in a warm (80F-ish watch for overheating), stress-free area inside?
To fulfill a fluid, and nutritional deficit you must acquire tube feeding supplies which consist of tubing, a large syringe, feed formula, and a scale. One must first correct a fluid deficit, then administer maintenance fluids as needed. You can find information in the link below pertaining to tube feeding.
https://www.backyardchickens.com/threads/tube-feeding-ducks-updated-5-7-2020.1211994/
If the duck is in a stable enough state, a physical examination may help suggest what might be the underlying problem. The owner will likely know any problems that may have predisposed the bird to develop into such as state, so if you know of any please do mention them.
https://www.merckvetmanual.com/exot...ltry/physical-examination-of-backyard-poultry