Your son can amend his return to remove the claim of exemption for himself.
Please be aware that this is a time-consuming process, as all amended returns must be paper filed. These take longer for the IRS to process than e-filed returns.
If your son amends his return, you should file a paper return yourself claiming him as a dependent.
Note - dependency is an all-or-nothing classification. He is either your dependent or he is not (for IRS purposes), so only one of you will claim him for the full year.
