This might be of questionable ethics, but if you put his twitter handle here, anyone googling for him will without a doubt come across your description. Google loves HN. Mention it on another high-traffic web site?
Send him an email warning him before with a chance to come clean or repair the damage he's done. Also, don't threaten; just write "I will post this on date X, and I'd like to hear your side of the story before then."