Are you, and your son getting any professional help? This sounds really serious.
Is your son able to understand consequences of behaviors? At 13 the interest in sex is understandable but the way it is expressing itself is troubling.
((((((big hug)))))) Is there a peer support group you might be able to use to share with other parents in similar situations? Would the policeman be willing to talk to your son about the potential criminal charges r/t this? (if it would mean something to him that is)
How is he doing in school? Friends? Sports? Other?