If your son is completing a tax return and he is the Taxpayer then the question is directed at him and him alone. If he personally did not receive the 3rd stimulus then on his tax return he would indicate that No he did not.
However, why is he not a dependent in 2021 when he was a dependent in 2020?
How old is your son? Is he a full time student under the age of 24? If not a full time student and age 19 or older did he have income of $4,300 or more in 2021?
