Use the amount of niacin (B3) in the B Complex. If there are 100 mg niacin in the B complex pill, then one pill to 1 gallon of water. if there are 200 mg of niacin in each pill, half a pill per gallon of water, or one pill in two gallons of water.
The duck can get better, most likely, if you...
I think he needs some rehab.
What is their food?
I think it would be more helpful for him to get as much swim time as possible. Supervised. No jumping so if there is no ramp in and out, someone needs to make them. B complex supplements. Might consider a brace - poultrypedia has a podiatry...
Miss Lydia covered it - I would like to chime in and emphasize that B vitamin supplements, time floating in lukewarm water (no soap, and supervised at all times) is very important.
Some folks have made hammocks for their ducks (they sit in them upright) to take weight off the legs for a while...