If you feel having him inside will help then by all means bring him in, if he gets too stressed then he might be better off with his buds. you'll have to make that decision when it comes. But most of all you want to get him over this and you may have to help him drink the niacin water by using a...
You might try moistening his feed with a little ACV and warm water. the ACV will help keep it from molding and also has some health benefits as well and sometimes they will eat it moist when they feel bad.
I hope you see improvement real soon.
Get the niacin make sure it's plain not time released or flush free, if a couple months old then 150-200 megs to 1 gal of water. daily. if you think a toxin then do a flush with activated charcoal It can be bought at Walmart or most drugstores in the antacid isle...