You could land up with egg on your face for allowing him to Skype you and doing it twice ... with webcams ... irrelevant that you may not have known at the time who he was, you allowed him to sex Skype you and were a willing participant.

He could try pull you down with him and say even though you're a mandatory reporter you knew full well at the time that he was at work / wherever he decides to lie about his whereabouts.

I just think there will be repercussions that won't stand in your favour / cause you a lot of personal grief over his disgustingness.

On the other hand, you'll put the suspicion of doubt in the minds of whoever you report this to, regardless of proof ever being found.