For now while you train him you can use a metal spoon with a licking treat to substitute for your hand, everytime they bite the metal, squeak or make a loud sound to show them that it's bad to bite. You can also try to figure out what he's afraid of and work with him on getting you used to that thing. If he's afraid of you, you can associate yourself with good by luring him to you with the spoon and when you pass the cage. If he's afraid of you touching him, you can use the metal spoon to distract him while you get him used to your touch, start with quick, small pets right when you give the spoon so that the licking treat is a reward, eventually move on to quick strokes on the back, then full petting. Does he seem to be afraid of anything specific? For example my rat Penny doesn't like going down the cage frontwards, she goes up and behind the cage, so I trained her to come down the normal way and it made her more confident altogether and less likely to hide.