Well, if you have a poultry vet, you could take him to the vet. If I were trying to assess one of mine with the problem, I'd start with checking the feet for cuts or bumblefoot, then check the legs for scrapes or wounds. Next I'd suggest carefully feeling the joints all the way up to the hip, trying to determine if a joint is out of place or a tendon is swollen.
Of course, a wound would need antibiotic treatment. A joint problem might need him to stay in water to support his weight -- which for me would mean bringing him inside somewhere and putting him in a shallow kiddie pool I keep for my ducks or in the bathtub. Treatment for a joint/tendon problem, short of immobilizing it which has it's own set of problems attached, would be hot and cold packs alternating just like for a human.
You might find he strained a muscle, and will get better in a few days on his own.
Healthy duck thoughts to you!
