You will not be able to e-file your 2024 federal tax return as his Social Security number has been accepted on another tax return. Amending his return will not change the IRS database for receiving tax returns.
You can print and mail your 2024 tax return now with your son added as a dependent. The IRS will pay you the tax refund on your return, if any.
Amend your son's return ASAP to change him as a dependent on his return.
