A PGP-Signed message is a way to prove that Satoshi is Satoshi, but however, I didn't see any message that Satoshi signed with a PGP key on it. Maybe those within his "inner circles" could confirm this one?
https://www.mail-archive.com/bitcoin-development@lists.sourceforge.net/msg06194.html
what if he really satoshi?
what will happen with bitcoin?
Nothing. He's just a user like everyone else ( But a user with a LOT of BTC ha ha ). It's not because that Satoshi say something that we have to follow him ^^
He's the founder of bitcoin but that doesn't mean that we should follow what he says (authoritarian control lol). Again, it is up for the economy to decide on which is which.