this will give you the dose by ageHow much do you think he can have in a day half of one ML or one full ML I mean he is tiny but he’s a goz
https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/
I'd stick with the liquid B complex since you have it and even sprinkle some Nutritional yeast on the feed once he gets eating good.
Hows Trudy doing?