Happened to me once when I banned a friend, so I can pretty much threaten to turn it to their local authorities right?
If you don't mind giving your router and modem (or HDD if you connect directly to the internet) to the police and let them look through it. Once they find evidence of a DDoS they'll submit a doe-subpoena to the ISP associated with the email address and notify the local police to formally charge the offender with violation of the Computer Fraud and Abuse act.
But if you threaten him by telling him you'll turn it into the authorities if he doesn't do X, that's called
extortion and it's a crime by itself.
