If you live in the United States, once you have enough information, you can use the IRS's whistleblower program to report him. You can get up to 30% of the penalty and tax money that the IRS collects if you wait until the idiotic budget sequestration is lifted.

Just make sure that you have solid information, because: "The IRS is looking for solid information, not an “educated guess” or unsupported speculation. We are also looking for a significant Federal tax issue - this is not a program for resolving personal problems or disputes about a business relationship."

http://www.irs.gov/uac/Whistleblower-Informant-Award