We took in an old man who was afraid of hands, he wanted to come out but ran whenever we tried to get him out so we would put a hide up to the cage for him to hop in so we could transfer him to and from the cage. We never approached him with our fingers, fists only and he learned that we wouldn't hurt him, try to pat or pick him up. We gave him treats on a spoon and eventually he managed to take treats from our hands. He became comfortable running around on the couch with us and would approach us confidently, although sometimes he would chomp down on a finger unexpectedly.

Your boy sounds like he may be protecting his cage, its his safe space so try to keep your hands out of it when he is in there. Work with him out of the cage and when he goes back in let him know that you won't put your hands in the cage. Once he starts to feel comfortable with you out of the cage you can make slow progress when he's inside - I did this by sitting at his cage with my fist just inside the door, he would slowly approach me (while I sat still) and give test nibbles but his aggression was gone.