The following morning (yesterday), it was looking just as bad. I picked him up and brought it inside, and started checking what else I had in the cupboards. A child's liquid vitamin would have been handy, but I didn't have any. Ditto thiamine injections. Finally I picked up my Spirulina powder, which we add to smoothie sometimes. Good amounts of niacin, thiamine, and other B vits. So I dissolved some in water, and spent some time slowly droppering the solution onto the little guy's upside down beak - he swallowed quite a bit little by little. After a while I concluded the stress of being held and away from mama probably was making him worse, so I returned him to the pen. I added more yeast to the feed, and put spirulina in the duckling's water bowls, also leaving them one bowl of plain water.
This morning, he is slightly tipping his head on an angle, and is not quite as steady as his siblings, but is way, way better than yesterday! He's walking forwards, keeping up with his siblings, and grooming himself. Today, I'm putting more yeast in the feed, and more spirulina in some of the water. I'm expecting a full recovery. :-)
In the case of newly hatched poultry, it is not likely a condition induced by their feed, or influenced by the parent's diet, but rather just one of those things who's cause is not well understood. It just seems to happen sometimes to one out of a batch. Prompt treatment will save the little one. No treatment means certain, unpleasant death.