Some individuals will show problems sooner than others. Maybe he isn't as good a forager, but if you want healthy ducks you will need to start feeding them some feed with more nutrition. This little guy is going to need extra supplementation real soon. Can you get to a Walmart for some...
Good questions, also what do you feed him? Sometimes leg problems are nutritional, if it's both legs then I would wonder.
How did you loose your other ducks?