Duckling stopped growing and got worse | BackYard Chickens

Are his feet turned inwards? Are his legs bowed at all?
It could be failure to thrive, but if he’s struggling to walk, I’d try treating for a niacin deficiency.
Is he the same breed as your other ducklings?

I’d get some duvert vit B complex, its in liquid form and you can get it from tractor supply if you’re in the US, 1ml over a treat daily for the struggling duckling.
Otherwise, you could look for flush effect (NOT flush free) niacin capsules, break the capsules open, and mix the powder into their water, it won’t harm the other ducklings. You can find this in some health stores, pharmacies or online.
You can also supplement to their feed as an extra to either of the above treatments, nutritional yeast can be sprinkled over their food, 1 tbsp per cup. This can be found in most supermarkets, or online. Ensure its nutritional yeast and not bread yeast.

Failure to thrive is something that can happen, but really this is a big unexplained label, we say the same thing with human babies, and it’s usually when something goes wrong after birth, or something is wrong genetically which doctors can’t fully explain.
So regardless of whether this duckling is failing to thrive or has something going on that we can’t see, personally I would give him a chance by treating for deficiencies and monitoring closely. It’s possible he will live and continue growing, but don’t get your hopes up