How do I say this...
Neon breeders do not seem overly concerned with keeping the genetic diversity up in their fish.
So often you get fish that will develop problems as they age. You'll hear many of us on the board laud Cardinal Tetras as alternatives because many are still wild caught (although they are being farmed now too) and the genetic diversity is still strong.
So what you may have is a fish with some whacked out bone or tumor problem. The type of thing that would normally get eaten first in the wild. But in your tank it lives an abnormally long life.
It's my understanding that a fish with NTD is going to die in a few days of symptoms appearing. So I'd add the little fella back into the tank. You don't have a school to protect him anymore, so any aggression may kill him, but you mostly have nice fish. I'd still add him back.