Get your taxes done using TurboTax

25% after the first 5 months then 1-2% per month after that.  

The IRS will get their money one way or another.  It may take a few years before they actually seize his bank and investment accounts but they can do that.   At the end, his executor will have to file the tax returns and pay the penalties when he dies, before any of the estate can be paid to the heirs.