Best way for him to prove who he is, is to sign a message using his PGP key. Though he may have thrown it out. I know I would have, since it would be too tempting to keep around, and any random newb coming in claiming to be Satoshi will be laughed out rather quickly. So, really, Satoshi can safely stay on this forum, since if he says he's Satoshi, no one will believe him. For example, I'm Satoshi   (Which is all something who was real Satoshi would say and want you to think.)