There seems to be some fuzziness over duplication of the $1400 stimulus. I'm of the opinion that your son should not claim it on his tax return, since you already received it. In Turbotax, he should indicate that he already received the $1400 even though he didn't actually get a 6475 letter.
For background, see this previous discussion, on the subject: https://ttlc.intuit.com/community/tax-credits-deductions/discussion/3rd-stimulus-dependent-eligibili... , which includes this detail from the law:
The 2021 Recovery Rebate (third stimulus) is in Section 9601 (Subtitle G) of the new law (American Rescue Act?) and creates Tax Code section 6428B.
Subsection (h) says:
‘‘(h) REGULATIONS.—The Secretary shall prescribe such regulations or other guidance as may be necessary or appropriate to carry out the purposes of this section, including— ‘‘(1) regulations or other guidance providing taxpayers the opportunity to provide the Secretary information sufficient to allow the Secretary to make payments to such taxpayers under subsection (g) (including the determination of the amount of such payment) if such information is not otherwise available to the Secretary, and ‘‘(2) regulations or other guidance to ensure to the maximum extent administratively practicable that, in determining the amount of any credit under subsection (a) and any credit or refund under subsection (g), an individual is not taken into account more than once, including by different taxpayers and including by reason of a change in joint return status or dependent status between the taxable year for which an advance refund amount is determined and the taxable year for which a credit under subsection (a) is determined.
