This is a federal beef, so you can at least file a complaint:
The Internet Crime Complaint Center. The amount is enough to hire a lawyer to send a certified letter. That can shake people up (sometimes). You could even pay for one of those Internet information searches.
A good PI can get nearly anything on anyone. Check to see if he's on probation. If you're lucky, he is and you can contact his PO. The main thing is to be persistent . . .