Am I being too hard on my son?

Long story short, I am the guardian of a slightly troubled 13 yr old boy. He is my best friend's son. He recently broke his girlfriends heart by sleeping with a neighbor girl. Yes I know he shouldn't be having at this age. That is not the problem. The problem is that he seems to view girls as property and not as partners. He goes out with girls he doesn't really like and then breaks their hearts...usually on purpose so they'll break up with him so he doesn't have to do it. A cowardly act on my part. So his punishment is to write a 5 page essay on relationships. He has written 3 pages so far and claims he has nothing left to write. I told him I was going to shave off his beloved hair tonight if he didn't finish it. Am I being too hard on the kid? I want him to learn a lesson.