I don't agree that IRS wants six years.  @sjrcpa is on the right track with wanting three.  But why not start with just one, 2024, and set up compliance starting now?  It can't get any worse.  And income tax is not his only problem, as pointed out.  Workers comp, payroll taxes, hiring practices.  

Your location suggests a cultural attitude about IRS and government that doesn't prevail elsewhere.  15 miles from an IRS Service Center and 15 minutes from the 19th Century.  Ask him how much he expects to collect in Social Security, with no earnings credits, and get a blank stare.  Ask the wife, maybe a glimmer of recognizing the problem.

And the wife?  Would you really condemn her to filing a joint return?