Aylesbury duckling limping

[Mollynoodles]

My six week old aylesbury duckling has developed a horrible limp. Summer has been fed on duck starter crumb and is now on grower pellets. He has also had tomatoes, lettuce and some mealworms, along with my four runner ducklings of the same age.

I examined his feet and they look normal. He didn't like me touching the top part of his leg though so the tenderness is there definitely.

What can I do to help ? What could he have done? Please help with any suggestions. Thank you for any replies.