If his refund is short $1,400 then that is because he said he did not receive the 3rd stimulus payment and the IRS has records that show he was sent the 3rd stimulus payment so they removed the $1,400 Recovery Rebate Credit that was entered on Form 1040 Line 30.
Go to this IRS website for his tax account to see if the 3rd stimulus payment is entered - https://www.irs.gov/payments/your-online-account
If he checked his bank account in the March - May 2021 timeframe and there is not a $1,400 EIP3 payment from the IRS then go to this IRS website for how to start a trace on the payment - https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-p...
