"Kick his donkey seabass!"?
That's one of the nicer ways I've heard it said.

A solid legal defence is the best offense here. Assemble any and all information related to what you did to prove it was yours (the backups, etc and the restrictions thereof) and give him the printouts. When he asks what this is all about, tell your tale.
Logic tells no lies, he must accept the truth. (If that's indeed what it is.)

Depending on his reaction, you can be a seabass, and/or walk away a winner, PSP ownership established.