A. Nope. It's simple: Grade-school buddies don't get to decide what's appropriate for your child. You do. Explain to him what the rating means and that there might be scenes in the movie that contain things he's just not old enough to see, or language you don't want him to hear. If he argues that his friends' moms think it's okay for them to see it, go with the classic line "I'm your mom, and I make the decisions for you."
You can help your son save face with his friends by giving him an out: He can't go to the movie because he's got a very special afternoon planned with family. Even if you don't go anywhere at all, baking his favorite cookies or playing a game together will keep his thoughts off what he's missing -- and you'll have the peace of mind that you stuck to your guns and didn't give in to the pressure of his peers.