Complaints filed in court are public record. So if you put it in the complaint, there will be others who will see it (at least the court clerk and judge) and any one in the public would have access to it. So doing that does indeed air the complaint publicly. The press and others routinely go through complaints filed in court to look for interesting cases to report on or to include in various databases, etc. You run the risk that your allegation would indeed take on a life of its own and cause him a great deal of publicity and scorn. Should that happen, you might find yourself facing a defamation lawsuit. And if you can’t prove what you say is true, you could find yourself liable to him for substantial defamation damages.
Two points on that. First, the statute of limitations for prosecution on federal felony tax offenses, including tax evasion, is six years. It is too late for the IRS and DOJ to criminally prosecute him for it. So he doesn’t have the worry of prison over this. Second, all the IRS can do at this point is assess the extra tax he owes plus interest and penalty. That would be expensive for him, but for the IRS to succeed, it needs to prove the amount of the unreported income. Simply sampling the contracts he awarded or had input on awarding won't prove anything about the kickbacks he was allegedly paid. Those kickbacks would not be on the state books, after all. The kickbacks would deliberately be done off the books between him and the person seeking the contracts. If they were at all smart, they likely did it in cash or other hard to trace means of payment that would not link him and the contractor bribing him. It is likely too late at this point to get any bank records for the guy to trace deposits that might point to unreported income. Ten years after the fact most records that might have been available to trace the income are going to be gone. Indeed, very likely the IRS will no longer have his complete tax returns at this point. If all you tell the IRS is “Joe Blow told me 10 years ago he was taking bribes to award state contracts and I'm pretty sure he didn't report that on his returns” that’s not very helpful to the IRS in tracking down the alleged unreported income. As a result, it won’t get much attention given how much work would be needed to develop the case and the likelihood that nothing would come of it because there would be insufficient records to work with at this late date. Had you reported that 10 years ago perhaps the IRS might have acted, and you might have got a nice reward for it, too. But I don’t see that happening a decade later.

