You can also buy human B-3(niacin) pills and add one to a gallon of water for drinking. Just don't get the "flush free" version. The sooner you get him on it the better his results will be. And yes swim time!
I throw a handful of nutritional yeast into my all flock or flock raiser a couple times a week to be safe, as a preventative. Seems likely yours will need that B complex since he's already affected.
I've also read to supplement with oats or wheat. Grazing is a great way to lighten the diet.