Big bar of coal tar soap, or any strong smelling soap, put that in cage first every time, and let him bite it, then make sure you wash your hands with the same soap, every time before you handle him, so he associates the smell with your hand, the smell will link to the memory of the taste, it usually works for me, one of the issues you see are people over handling them, ie for too long, they get pissed off, being held in one place, they can be busy little animals and some get bored easily, if that fails, the alternative has already been mentioned,