Generally speaking, parrots don't apparently use violence against other flock members in the wild, so your Grey simply will not understand your use of violence against him. Odds are good he will never be able to trust you again. There is much outdated and incorrect advice being given to people about biting parrots. So once again, in our efforts to give negative feed-back to parrots, we have only succeeded in rewarding them. Actually, it is quite simple.
Parrots are extremely empathetic creatures who watch our facial expressions closely.
He will understand your displeasure if you give a tremendously dirty look. Then, make him step from one hand to the other over and over, while you keep saying UP in a very firm, but not loud, voice. Do this several times in a row and you will be amazed at the difference. If you are firm and consistent, reminding him of this will put him back under control. And without the positive feed-back that he inadvertently received before, the biting should end.