@Karen R - just to confirm, what was on Line 30 of your son's Form 1040?
did he check the box that someone CAN claim him in 2021?
If you were claimed as a dependent on someone else’s tax return for 2020, you were not eligible for the third-round Economic Impact Payment. If no one can claim you as a dependent for 2021 and you are otherwise eligible, you can claim the 2021 Recovery Rebate Credit, and must file a 2021 tax return to claim the credit.
