I think what he is saying is that to prove him out you would need a het axanthic or axanthic that is the same line. Breeding a possible axanthic to a normal will just give you a bunch of possible hets (they will all look normal). So there would be no way to tell if he is actually axanthic by breeding him to a normal. I mean, you could, it would just take a few breeding seasons and to be honest, you will probably end up with a lot of normals.