Welcome to the Duck Forum!
So it sounds like you just got these ducks, right? It may have been this way since hatching. Something you can do that will probably help at least in some small way is give this one as much swim time, bath time, float time as possible, to get the weight off the leg, increase circulation and strengthen muscles.
At this age, it may be difficult to do much, but a good regimen of vitamins, electrolytes and probiotics will boost the duck's immune system and help whatever ails him.
I would do my best to be patient and see how far along I could help the little one. Check in on OldGuy43 and find out about Sunsinger.