HELP! Leg problem.

Hello everyone. I have a Cayuga duck that is about 7 weeks old. He has a very hard time walking. His hips appear to be extended, he cannot stand up straight and often walks on top of his own feet. We've been giving them niacin in their (non medicated) feed since we've had them. I've been giving him some swimming therapy to try and strengthen his legs. He is now at the point where he just lays down. He is also very light (compared to the other ducks) and becoming lethargic. I really want to give him every chance I can so I need to hear every idea out there. Your help is greatly appreciated!