Hey Robo, I think in order to get an anwer, the professionals here might require a little bit of information.
Size of tank, how long it's been set up, how it was cycled, what the Ammonia/NitrIte/NitrAte levels are, if there's been any change in pH, and any other fish in the tank.
However, Neon Tetras are prone to short lives and illnesses unfortunately. That's especially true when bought from breeders of an unknown source. Have you read up on Neon Tetra Disease?
Also, if the tank is in ideal conditions and he has no real physical signs of disease (curved spine or spots or legions or difficulty breathing, etc., other symptoms)... it could be that the Tetra "Tribe" has "spoken" if you will, and he's an outcast or very shy. This might mean he isn't able to get to as much food. But please make sure you've ruled out the above.