Niacin can come in several forms. The Nutritional Yeast vs Brewers Yeast stickied thread is a good read-through (I use Nutritional Yeast which I can get at our bulk foods store, but it can also be found in health food stores/sections or online.) I put a couple tablespoons in with a couple cups of food every day.
It's also part of the Vitamin B complex family - you can try dissolving the capsules in water or you can go the liquid route. I use the liquid on treats instead of trying to jam it down their throats or wasting the capsules in water they might just spill out. There's some good threads on that as well (what kind, how to administer, dosage)
If he's not eating right now, however, the jamming it down his throat option would be the only way to get it into him efficiently, and if he's on his third or more day of not eating anything then you've got bigger problems than just a niacin shortage.
I know you're working on this alone and you're busy and it seems very stressful for him as well, but you've really gotta just focus and get him cleaned all the way up - going little by little each day leaves room for infections and untended wounds to set in and worsen. Sit on the floor of a small quiet room like the bathroom, burrito him up good in a big towel, hold him in your lap/between your knees and get the feather clipping, wound cleaning and treating, and maggot clearing all the way over with. Once it's totally done then it's done and he can begin to really rest and heal then. Right now he's in a really uncomfortable and malnourished limbo.
Keep supplying him with wet/softened food, ducks do a lot of work with their necks/throats when eating dry stuff and if he's sore that'll be the last thing he wants, so make it as easy on him as possible. I know he's not going for it right now but the hope is he eventually gets hungry enough on his own. You could also try mashing up some banana, peas, watermelon , anything mushy - they're not as healthy as his actual food but eating anything is better than none at all. If he continues to not eat anything you're gonna wind up in tube-feeding territory.