At the current time short of a message from one of his accounts or a signed message from one of his keys there is no way to know for sure if he is Satoshi. Even if he was it would be hard for the government to force him to pay taxes on Bitcoin considering they cannot definitively prove he owns x amount of coins .
Unless the ecosystem around bitcoin is build, the owner eventually will need to exchange for fiat realize the fortune. So maybe government should track the account regardless who is behind the account.