Please see this IRS webpage for information on how to respond to their Notice CP567. You can call the phone number on the letter, or you can make an appointment to visit the Taxpayer Assistance Center again for help with resolving why the ITIN for your son was not approved. If the ITIN issue is resolved, then you may be able to claim the Credit for Other Dependents.
If your child does not have an SSN that is valid for employment, you may be eligible to claim the Credit for Other Dependents for that child once they have an ITIN.
The Credit for Other Dependents is a tax credit available to taxpayers for each of their dependents who was a U.S. citizen, U.S. national, or U.S. resident alien in 2021 but can't be claimed for the Child Tax Credit. The maximum credit amount is $500 for each dependent. But the credit can only reduce your tax and provides no benefit if you don't have any tax.
Please also see this TurboTax tips article regarding filing with an ITIN.
[Edited 03/27/23| 11:32am PST]
@Mobeen (edited)
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