You are not required to do so, but he may be required to mark that he CAN be claimed as a dependent. He must mark his return that he can be claimed as a dependent if he does not provide more than half his own support. The IRS provides a worksheet to help determine support.
If he can be claimed as a dependent, even if he is not actually claimed, he is disqualified from the recovery rebate credit that he would get from not receiving a stimulus check, but he still may be able to claim the American Opportunity Credit. The IRS has a special carve-out for this credit where you can claim it even if you are eligible to be claimed as a dependent as long as no one is actually claiming you as a dependent.
Even though he would not get the stimulus, the benefit he receives from the American Opportunity Credit may outweigh the additional tax you would pay for not claiming him. Remember to look at the whole picture, including your state return to make this determination.
