I speak as a father of three boys and two girls, ages 20 to 8.
There isn't any possible excuse for this behavior. No set of circumstances, even a gun pointed at your son's head, can justify this. No woman or girl deserves to be treated like this.
If he's under age 18:
After a sincere and calm discussion of the heinous crime your son has committed, you should calmly take a wide and heavy leather belt and administer 10 to 15 full-force whacks across his underwear-clad butt, with your wife there to witness.
Then he should lose all outside the home privileges for one month. If he has a job, call his employer and tell them he won't be there for two weeks.
In that time, he is to write an essay on the dignity of women and the evils of abusing them, with side references to the evil of strong people overpowering weaker ones.
Then you should arrange a meeting with the girl's parents, at which time you will accompany the young miscreant to the appointed place and he will apologize for his deeds.
If he's over 18, hand him over to the local police.
Don't like my opinion? You asked for it!
{edited to add:} [b]OOOPS,[/b] Sorry, not your kid. Didn't see the age reference earlier. But, I stand by what I would recommend for punishment [b] regardless of whether the girl sez she was playing or not.[/b]