I was in a similar situation with my first ducks being Pekins. I wish more instruction would come with them! They are unfortunately bred to put on weight super fast as they are meat ducks, and their poor little bodies often can not keep up :(. The liquid vitamin B complex is like a rescue med...
If the hocks are swollen it may be a slipped tendon. I have no experience with that but you can find threads on BYC about that issue: https://www.backyardchickens.com/threads/pekin-duckling-developmental-issues.1578595/page-2
Sweet babies! Are his hocks swollen? Are his legs and toes just pointing in, called pigeon-toed, or is the foot actually twisted - as it appears to be in the photo? If the foot itself is actually twisted you need to devise a little brace to flatten the foot - you can find other threads on this...