Don't confess, just don't use the letter, If you confess, there's no way he will write you a good letter, and he might send a bad one.
If you use it, there's a chance he will talk about your great mcat score. Anyone reading that will discount the letter, so it won't help.
Or, he might mention the 33. Of course, they will assume he heard or remembered wrong. But what if they ask you? You will lie and say you told him 30.
But now you have drawn undue attention to your application and your mcat score, and perhaps someone will think you were lying.
So, this letter can't possibly help, and could hurt. But if by "your doctor" you mean either your personal or family physician, that letter is worse than useless anyway. If it's a doctor you shadowed with, it's almost definitely useless. So don't use it, don't worry about it, and stop lying. Not only is lying dangerous, but you're obviously not good at it.