He won't have to pay it back.
The IRS recently acknowledged that "there is no provision in the law requiring repayment" of a stimulus check.
The way the law is written, stimulus checks are actually just advanced payments of a new "recovery rebate" tax credit for the 2020 tax year.
If your check is less than your 2020 recovery rebate credit, the law specifically says you'll get the difference when you file your 2020 tax return next year.
Thankfully, the IRS has provided guidance. You won't be required to repay any stimulus payment amount when filing your 2020 tax return, it says.
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