A good duck vet is best, not always possible to find.
Meanwhile, I would add some B vitamins - can't hurt. Lately I have been thinking that B Complex might be the way to go. Use the amount of B3/niacin as a guide. 150 mg B3 per gallon of drtinking water.
And I would consider building a small sling setup to enable the duckling to stand in the proper form. I would give supervised tub time with the water just at the top of the legs, and brooder temperature, for a few minutes two or three times a day. No soap. Constant supervision, and dry off with a washcloth afterward.
I would compare the problem leg to the good one and look for clues - tendon, bone, something else going on?