Sounds like your duck may have a virus. Does he have diarrhea too? Are his droppings watery? If so, he could be coming down with duck viral enteritis. Supposedly duck viral enteritis is incurable and fatal, but I would try giving him a nanosilver solution to drink for at least 2 months and then every night for maintenance. You can buy Silver Biotics nanosilver solution at most of the better health food stores. It's cheaper to order it online, so you could place an online order to buy more when your bottle runs out.
You would need to take away all your duck's water sources and have him drink the nanosilver solution on an empty stomach and wait at least 15 minutes to a half hour before he eats. Give him straight nanosilver solution to drink all night (put a couple tablespoonfuls in a little ramekin next to him), then have him drink it at noon, in the afternoon, and around sundown. Have him on this protocol for at least 2 months, then just have him drink it at night for maintenance dosage. Don't worry about how much of it he drinks, but if you're wondering, the maintenance dose is probably about a quarter teaspoon a day for a healthy duck. A sick duck needs much more.
Hand raised ducks that are not vaccinated against viral enteritis and viral hepatitis are susceptible to getting these diseases, and some of the first signs of them are when your duck gets weak and stops walking. He could stop flying next. Don't delay, buy the Silver Biotic today. Nanosilver solutions have no problems with other medications and no side effects...but they must be taken on an empty stomach. Supposedly they are very effective against any viral, bacterial, or fungal infections.
Please keep us posted on your little one's progress.