If you pull your hand away and yell, then your parrot gets a reaction out of you and will try again to get the same reaction. They think it's a game, try & show no reaction to him,try to distract him when you can see that he's going to bite,you can gently wobble your hand if he is on you this works well for me.I also have a clicker ( like some people use for dog training! ) if mine go to nip i click on the clicker & say a firm No in a strong voice, the clicker distracts them.
Some people use time out, if your parrot bites you simply return him to his cage for 5 minutes & walk away from him & say nothing.Then let him out as normal,back to the cage if he bites & again dont respond to him.