Ha, you're fucked. Someone who is a bit messed up in his head is now obsessed with you. There's delusions of grander and he is probably thinking the FBI, police, CIA, whoever is secretly monitoring his Youtube account even though the videos are private to get more leads off of him. After all, he's the only smart one and everyone else is either the enemy or too stupid to follow his leads. The world is relying on him to catch the killer!
But seriously, I would be wary of pushing him too much. I wouldn't think it would be much of a step for him to get a gun and come knocking on your door. He's dead set that you're the killer.
I'd guess it was a wonderful prank, but I found some archived posts from 2005. Four years is way too long to be staging a prank. Unless you did something really horrible to him.