Hmm, Im not sure. With Neon Tetra Disease usually has a white stripe, like a band around the fish. Do you have Primafix on hand? That would help if it's bacterial and combined with Primafix works better. You could add some salt as well. Keep the tank nice and clean and hopefully the little guy pulls through. I would hate to add stronger meds just because you don't know what it is. That's the problem with fish sometimes. I just wish they could tell us what hurts, LOL.
The link below has a picture with Neon Tetra Disease which may help.
http://www.fishpalace.org/Disease.html