Bright and perky but cannot stand, or all better?
Welcome to the Duck Forum.
Some possibilities:
Viral illness
Bacterial infection
Injury
Poisoning
Regardless of the nature of the problem, a little warmth, vitamins with electrolytes and probiotics, fresh water, clean bedding, and TLC are always a help.
If you can get him into lukewarm water deep enough to float in, you can observe whether he can move his legs at all, whether he has any balance.
If it is poisoning, it could be something like botulism. If that is the case, you want to carefully flush his system with an Epsom salts or molasses flush. Don't overdo it. There is a sticky with instructions for a flush.
https://www.backyardchickens.com/t/256233/ducks-sticky-topics-index look toward the bottom of the list.
Is there anything yucky in the pen he could have gotten into?
Also, some ducks are more sensitive than others to toxins. Check the feed. What is the feed you give them? Lots of corn? With the drought, there have been high levels of aflatoxins in some crops. That stuff is deadly.
If it's viral, just keeping him warm enough with good food and water and vitamins etc. may be all you can do to help his immune system fight it off.
Has he had any trouble with bumblefoot?
Could something have fallen on him, or could he have fallen?
What other animals are in the area? Any chance a predator tried to nab him? Look for puncture wounds around the neck and under the wings.
And please keep us posted. I must go run errands, but I will subscribe to this thread.
Praying.