What feed are you giving him?
Waterfowl need more B vitamins than other birds, often feed formatted for chicks isn’t enough for gosling or ducklings and issues like this “vitamin deficiency symptoms” can appear. Sometimes even the right feed doesn’t always have the right concentration so supplementation is needed.
If you can find a liquid B complex and a liquid multivitamin start adding a few drops to his mouth a few times a day.
If you are in the U.S durvet liquid B complex and Poultry Nutri-drench are good sources, the durvet B vitamins will say injectible but it can be swallowed too.
Nuerological issues are typically a sign of too little vitamin E and B vitamins like niacin, thiamin, and pyridoxine, other vitamin or nutrient issues like not enough protein can cause issues too.