There are all kinds of ridiculous advice here, some leaning on beating up the kid. I might as well throw a ridiculous advise out.

Perhaps the poor kid is just freaked out to tell his parents what had happened. Perhaps the kid is afraid of getting his butt whipped at home. No need to create animosity with the kid. Cut the kid some slack.

One thing I should point out- When I was a kid that age, I made sure I settled my scores one way or another. Mature thinking at 13? Buahaha. You better hope this kid isn't like that.