Tax question re: claiming a dependent

My 18-yr-old son filed his taxes but selected the box that said no on could claim him as a dependent. On my federal taxes (as parent), I claimed him because he met the qualifying child requirements (lives at home, full-time student, we pay for everything, etc.). My taxes were rejected because his were already accepted and his social security number had already been filed as a taxpayer, rather than a dependent. So, I amended his (selecting someone else could claim him as a dependent) and it was accepted. How long should I wait before resubmitting mine, claiming him as a dependent? I'm getting down to the wire...only 7 days left. Thank you!