If when he filed his tax return, he indicated that he is claimed as a dependent by someone else in the Personal Info section of TurboTax, then there is no problem. You can claim him as a dependent and e-file normally.
If he did not indicate that he is claimed as a dependent by someone else, then your tax return will be rejected. In this case, he will have to amend his return to indicate that you are claiming him. The IRS will take 12 to 16 weeks to process his amended return and you cannot e-file during that time.
I recommend that you claim him as a dependent and file by mail. By the time the IRS processes your paper return, they would have processed your son's amended return and everything will match.