Get his IP address from Youtube -> Last IPs to log in. Then search for an IP locator and you will have his exact street address. Then you and your friend can grab your glocks and f*ck him up.
Just kidding, there's no way to track that. Perhaps his password was easy to guess, or his security questions could be answered by someone who knew him well with a little bit of social engineering. Either way, he messed up and is paying the consequences. Live and learn.
Good luck!
Jared