Well dear there may be so many problems that made him sick . Either you have crossed breed the blood related ducks, or might have taken the first layed eggs for incubating. I won't tell you to give him a mercy killing, but give him a full protection until he dies.

Euthanasia would be the kindest thing to do.

So all my ducks are growing except the one with the bad leg. He's only gotten worse and is walking on his knees now instead of his feet. He still eats and drinks water and all that and we've been helping it swim everyday as well as giving it foods with niacin in it everyday to see if that helps. His legs still bend but walking on his knees causes him to fall on his back then he can't get back up.

What should we do? Should we put it down or is there another way to fix this?

none of the other ducklings have this problem.