Your son would have to amend his tax return to change his dependency status to being a dependent from not being a dependent. An amended tax return can only be printed and mailed, not e-filed. The IRS will take up to 4 months to process the amended tax return, or longer.
Until he does that you will not be able to e-file a return with him as a dependent on your tax return. After he files the amended return, you can only mail your tax return since the e-filed return will continue to be rejected until the amended return has been processed.
