The problem is, in order to tax him, you would need to tax every other american mega-billionare in his tax-bracket, and that probably doesn't sit well with recurrent SUPERPAC donors which legally influence your politicians with loads of cash for their political campaigns.

Berkshire the company is one of the US's biggest taxpayers. Buffett himself is giving most of his money to charity (the Gates foundation).

I'm not sure why this is getting downvoted. As you say, Buffet himself is asking to be taxed more. It's not really relevant to say that he's giving it to charity here. (Although I think it's great that he is.)

Billionares say things like that all the time but that's just for publicity. Truth is, if he really wanted to be taxed more, he could. There's no need to change any laws for that. His Berkshire Hathaway is already claiming a lot of tax breaks to lower their tax liabilities - if he wants to contribute more to federal budget he just needs to stop doing that :) After all, claiming a tax break is a privilege, it's not mandatory.