I don't really understand what you're asking. It seems like you have made your mind up already.
You're not going to the police, we've established that. And, to be honest, I don't blame you. It's really just more of a hassle for you. If he does it and gets caught, then it's his fault. It's highly unlikely the police will do anything more than speak to him if you went anyway. Nobody actually got assaulted and nobody has made a complaint before, so there's not really anything to pursue, especially since it happened so long ago.
I assume he must have told you in confidence, so I imagine he does feel some sort of guilt or he wouldn't have bothered. You don't break a confidence like that by snitching. Nobody likes a friend who has a big mouth. If I was one of your other friends and I heard you immediately started spreading it around, I wouldn't trust telling you a damn thing.
Yes, advise him on his conduct, but apart from that just be a bit more wary. As long as he does keep to his word and never does anything like this again there are no problems.