It looks like somebody else claimed your son as a dependent for 2020. If you are sure that he has not filed a 2020 tax return, then someone else claimed him. Maybe he did file and didn't tell you. Could his other parent, or some other relative, have claimed him?
In any case, you cannot e-file your tax return this year. You will have to print it, sign it, and file it by mail. Eventually you and the other person will get letters from the IRS to straighten out whether you are entitled to claim your son.
