I am almost certain your duckling has slipped tendons, and it's common to larger-breed ducklings, like Pekins. YOU MUST have the tendon slid back into place and wrapped/taped for support, IMMEDIATELY, or else this condition will be permanent and your duck will never walk. Slipped tendons can only be repaired very soon after the slip, so time is of the essence. Since it has been a week, you need to hurry! Do you have an avian vet near you? Or a vet school?