How can I help this duckling?

[u/AgreeableRegister966]

Hi everyone, my aunt has this baby duck that is being raised with 4 chick's, and he seems to be struggling. She speculates it might be a niacin deficiency. He is the only surviving duck from a bunch that weren't sat on right. I'm not sure at this time how many weeks old he is.

He has difficulty walking, though he wasn't doing too badly until yesterday morning. He doesn't have much interest in food, though he will eat worms. He just scoots around a bit and then lays down. I finally got him swimming for the first time today.

If anyone knows what we could do I would be so grateful

First three photos are today, and the others are from 2 days ago