Struggling Duck...Please help...Ty!

Hello all,

We have 5 ducklings that are about 7 weeks old... the only male, a cayuga can hardly walk... he can stand for small periods but his hips are wider than the rest and he steps on his own feet... he is having starter/grower feed with niacin added. Anyone knows what this is and how we can help him...he feed him separately bc he started to get lighter than the rest and he gained weight back...my husband puts him to swim in warm water as physical therapy but I don't see improvement. Thank you for the help.