he will have to amend his return to change from claiming himself to indicating he can be claimed as a dependent on another person's return.
assuming he's your qualifying child (i'm ignoring tests he would need to meet based on what's in your post) - resides with you when not in school; isn't married and filed a joint return; and you contribute over 1/2 his support- you can claim him, but will have to mail in your returns.
if he doesn't amend, the IRS will eventually get around to sending both of you a notice asking to amend. if no one does, another notice follows asking each of you to prove you're entitled. the IRS then makes a decision based on what's submitted. the winner gets to claim him. the loser usually gets a notice assessing taxes, penalties and interest
