IRS tax transcript.

https://www.irs.gov/individuals/get-transcript

A transcript is the record of his tax, not just his tax return.  He might make a fake high income tax return to fool you.  But if he make a fake high income tax return to fool IRS, and that tax return is entered into IRS's system to become tax transcript, he'd have to pay more tax.   So a tax transcript is unlikely to be fake.