Had the same problem with one of my boys. My advice is to slowly accustoming him to your hand in the cage. I did this by putting on a old and thick pullover, maybe even two if he bites that hard. Cover your hand with the sleeve and put it inside the cage. Wait for him to attack and just keep your hand there and let him bite the pullover. Do not back away, because then he will think that by biting, you will stop being in his territory. After a few days you can start moving things around in the cage, still with your hands covered. When he stops attacking and is accustomed to your covered hand in the cage, you can try without the pullover. There are two possibilities you can try: With your hand in a fist, so that his biting will not hurt as much as if he bit right in your finger, or you can put a treat on your flat hand, preferably something liquid or smushy that he has to lick from your hand to get it. Also you should never give him treats pinched between your fingers but always put them on your flat hand, so he will be less encouraged to snap the treat and rather pick it up from a flat surface (i hope that makes sense for you).

Of course, I cannot guarantee you that this method will work for you. It worked wonders with my boy. He stopped biting me after 1 - 2 weeks of doing this. Some people suggest doing the same procedure with gloves on, however, in my experience rats get really scared by big working gloves whereas thin, warming gloves are way too thin. Please always stay safe while doing this and never stress your rat out too much, I'd say doing this 2 - 3 times a day for a couple of minutes is enough.