I can definitely understand not wanting to talk to him about such an uncomfortable subject. Hell, last week I had a hard time explaining an innocent question from a teenage cousin who while watching a crime show asked what sodomy was. Great idea contacting Dlist, but if he's lying about there, he could be lying on the other dozens of hookup sites, and may also be lying in real life. You still need to talk to him. If he is gay or bi, then he probably has all sorts of questions but no one to answer them. Not trying to sound corny, but you could change that. If you can't muster the strength to confront him on person or on the phone, send him an email and tell him what you know and why his behavior is dangerous in a nonparental way. Then ask him to contact you when he is ready.