Sounds like you may have had some troubles in incubation to have these duckling hatch with these leg issues. Slipped tendon etc. They really need some Liquid B complex [TSC} carries one it will say injectable but we use it orally 1 ml per duck over a yummy treat.
At 9 weeks some of these leg problems won't correct themselves totally but can still help.
With Pekins most likely they will need to be supplemented with niacin their whole lives. Nutritional yeast is another good source of niacin, but the ones already having leg issues need the liquid B complex. This little one with the locked leg is most likely a slipped tendon which won't correct itself now without surgery but there are other options too like wheelchairs for poultry and slings to keep the duck from injuring itself. We have info on these if interested. info
https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/