Vitamin E can help neurological problems, but it may take awhile. A lot depends on what has caused this. You may need to keep tubing him if he refuses to eat on his own. I don’t know much about their digestive system, so others would know more.
Sorry about your loss and the illness in the brother duck. Could he have been very dehydrated or possibly injured by another ducks. Do you ever see any bullying? I really know very little about ducks, I will tag @Miss Lydia. I would suggest vitamin E 400 IU given orally daily for a couple of...