Worried about my duckling

[macyduck]

I bought 3 ducklings last week, two of which are more than twice their original size now and one that hasn't gotten any bigger. They were all healthy seeming up until yesterday when we took them outside to play, our little runt wasn't walking with the others and when he tried he limped. He keeps his leg curled up and doesn't move much. I put him in the bath and he swims perfectly! His size also concerns me so I take him out and hand feed and water him daily to make sure he's getting plenty to eat. He has chick started (non-medicated) and grit. He also doesn't stretch his neck like the others.
I'm not sure about their age but I'm assuming they were 3-4 days old when I bought them and believe they are Pekins

I uploaded some short videos on YouTube but "I have not been approved to post outside links" so it deleted my other thread.
My YouTube name is Macy Crabtree

 

[sassysarah]

Maybe he is a different breed looks a little darker then the other two. Where did you get her?

 

[macyduck]

Maybe he is a different breed looks a little darker then the other two. Where did you get her?

I bought them all at tractor supply co. And they said they were Pekins. But you never know with stores like that! But the one that's grown the biggest has gotten lighter it seems or maybe I'm just imagining that!
Even with her/him being a different breed possible, I'm concerned about the leg