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 .
This triggers another question. Can the government force to freeze any account if it has legitimate reason to believe that the account holder is a US citizen and therefore asks to build in the open source bitcoin client software certain rules to refuse to transact with the account until income is filed. Just like the hacked bitcoin from mtgox.
The bigger question is, is it desirable?