Try introducing your hand with something that he likes to eat in your hand, then when he's busy chomping down gently pet his shell. Try to avoid touching the face until he really trusts you. I have 2 Russian tortoises and they went immediately into their shells if they saw our hands coming towards their face. Now that my girlfriend and I have had them for over a year now they don't mind having the top of their heads pet if we take a slow approach to their faces. Once you and your turtle have gained a trusting bond then the biting should stop entirely, but until he is comfortable with you, don't freak out when he bites you. Just stay calm and it'll help him trust you better. Turtles and tortoises both are frightened by fast and loud reactions. So yeah just offer him something he likes to eat and slowly move towards his shell once he's taken the food.
