At the same time, there is nothing stopping him from handing the government the difference between what he has paid in the past and what he would have paid with the typical tax rates he's talking about. Talk is cheap.
It depends on your objective. Is your objective to take money from billionaires? Then, sure, he could just give it.
Or is your objective to fund the federal government? Mitt Romney doesn't have that much money. (All the billionaires together don't, at least not for long. But they go much further than Romney alone would.)
It’s nice to see someone who knows about, and uses, the loopholes advocate for closing them.