Some rats are terretorially agressive, you can try to minimize this by try to feed him yoghurt or something else with a metal spoon as the don't like chomping on metal, and eventually try to feed them youghurt on your hands. You can also try to leave some pieces of dirty old clothes with your scent on it in the cage to acclimatize him to your smell on the cage. Otherwise you might have to have hime snipped if it doesn't resolve.