Sometimes neons tetras come down with a strange "neon tetra disease" where they pretty much just stop eating, lose color, and die mysteriously. There is no known cure for it, and I have heard of it spreading to other tetras in the group. I would isolate him. Although you could try treating him with an antibacterial just in case, it may not work--but at least you tried. Do could also do a large water change in case it is pollutants causing this.