Late to this post but I don't think this is as serious as people are making it out to be. It's likely a bacterial skin infection and similar conditions posted previously on here seem to suggest it has little effect on overall health and will continue to spread on the face and underside of the body over several months, if left untreated.
You'll need to visit a vet for antibiotics, yes. However, I don't think the skin condition is life threatening. He is a bit on the thin side. Check the diet to assure it's suitable for axolotls. Look into wax worms to fatten up.