I had 15 eggs in a home made incubator.
I know exactly what your going through. One of my little guys hatched first, way toooo soon. He hadn't absorbed all his yolk sac yet. I gave him as much care and attention as I could. He looked nearly like what you have with your duckling. He acted the same way when I tried to get him to drink water. Would do it if I dunked him, but not on his own. He was always tired, lethargic.
I kept him warm and comfy, but unfortunately he lived for 2 days and stopped breathing last night.
If that duckling will eat, then I suspect he will be just fine. However, two of mine were not properly developed and would not eat. That is how I knew something was wrong.
I could never have the heart to cull him, but that's just me. Just give him as much love as possible, and whatever happens happens. If it is meant to be, he will pull through.
If not, then it wasn't meant to be.
My fingers are crossed and I'm wishing your ducky a speedy recovery.
Keep us updated.