You need some proof. You can't report it to your local police unless you know a local teen is involved. The perverted-justice site may be of some help. Talking to someone in your local media may also help. If you have a local FBI office, you might try contacting them (again as long as you have proof).
There is nothing wrong with an older person moderating a teen site, I was moderating a chat room for teens at one point. I remember putting one kid on a time out for talking about bare naked ladies

But if he's using his moderator status inappropriately that's a problem.
One other thing you can do is write to the site ownership. Tell them that since they have been notified of this guys actions and have failed to take action, then they are opening themselves up for a liability suit.