Have you checked his footpads for signs of bumblefoot (pododermatitis), does each joint seem to bend correctly? Is there any swelling, or discoloration on the leg?
With Pekins, it is pretty common for them to develop limps, that is often due to soft tissue trauma, or sprains, but infections of the bone, viral arthritis, and gout are among other possible suspects. Treating the problem as trauma-related is a good start, and if he doesn't show improvement that, you can consider more serious problems.
Reducing further movement in the leg is vital for proper healing, and can be achieved by setting up a small pen for the duck, and providing easily accessible feed, and water dishes. Reliving weight off the leg can also be beneficial in the healing processes, for that, you can let the duck swim in cool deep water about once a day, and occasionally adding Epsom salts to help further reduce any inflammation going on. B vitamins, especially niacin, are important for leg health, you can add some to his feed or water using either nutritional yeast, 1/4 of the plain B complex, or a few drops of Cattle B complex. Doing all the suggested treatments above can promote healing, but in the long run, time, and rest are often needed for complete recovery.
