No. You will not have to pay back the $1,400 for him.
The third payment is based on your 2020 return so if he was your dependent in 2020 then you are entitled to the payment. His status in 2021 does not matter for the third payment. At this time the third payment is the last one.
Finally, even if there was so type of mistake, the IRS has said you will not have to pay back stimulus money if you otherwise qualify for a payment, i.e. if you are a U.S. citizen or resident alien.
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