@Miss Lydia and the others are WAY more knowledgeable than me at these things.
I defer to their knowledge.
All I can say is his little leg certainly does look odd.
The Durvet is the best option as stated by others.
Nutritional yeast is what I used when I had ducks a few years ago. I got it in the bulk section at Sprouts but also saw it in the vitamin area of the main grocery stores.
I used it from day 1 and not with Peking ducks. They seem to need more...
I will tag a few that may be of more help with the one that has trouble walking.
@Miss Lydia, @Canadian Wind would either of you be able to help?
Duckling (Peking) age 5 days, non ambulatory for the most part, falls, has spasms. Other duckling same age thriving.
Looks like you are using the clip it came with. Do not trust that clip.
Add some chain to secure it please. It's also better to have it hanging pointing down but I don't think your set up is capable of that.
You will need a much bigger brooder soon. They grow extremely fast.
The water dish...
Can you post a picture of your brooder? They need Heat but also room to move away from the heat.
Heat lamps are a potential fire hazard. Pics will help to see how you have it set up so if it's a danger you can get advice on better securing it.
They are to young for peas or watermelon. They need chick starter and some nutritional yeast or brewers yeast sprinkled in for added niacin that ducklings require.
Please stop all treats for now.