If your son filed as independent last year (and shouldn't have), you are correct that he would need to Amend his return.
If he did not do so, then for 2021 it depends on who files first. If you file first and claim him as a dependent, his Efile will reject later.
If he files first and claims himself, your Efile will reject, but you can still mail in your return and claim him as your dependent.
The IRS may contact both of you to determine who filed incorrectly.
Click this link for more info on Claiming a Dependent.
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