If what he says is a joke, and he's a troll, then he's also an unreliable witness. Hence why a trial is unnecessary to find out the truth? Because how can one know that he's telling he real story now?

Really, if you confess to a crime, and then turn around and say "ha ha, only kidding!" don't be surprised if people find it hard to trust anything you say.

Note: I'm taking what you've said here at face value and otherwise know very little about this case.