Are you in the UK? hopefully you can find plain niacin but the best would be B complex in liquid or tablets. The tabs you can crush up if you can't get it in liquid. We have seen so many duckling helped by using B complex.
Also check your bag of brewers yeast some don't even have B3 in it. But...
Can you tell us what niacin you are using asking because flush-free and time-released will not work?
Glad to hear he is still eating and drinking. That gives you hope.
I looked up wobblers syndrome and couldn't find it affecting any animal other than dogs. Is the vet you went to an avian vet that knows about poultry?
I'd still try by giving the little one either poultry cell or poultry nutri drench it deserves a chance.
Is the duckling eating and drinking?