You are looking at it the wrong way around. You are never required to claim someone as a dependent even if they qualify. But the important question is on your son’s tax return; he will be asked “can you be claimed as a dependent by another taxpayer?” If he can be claimed, he must answer yes even if he doesn’t want to be claimed and even if the person who could claim him agrees not to claim him.
You can claim your child as a dependent if they do not provide more than half their own support, and if they are a full-time student under age 24, and if they live with you more than half the year. College students are normally deemed to live with the parents even when they are away at college because college is considered a temporary absence.
For the support test, student loans taken out by the student in their own name count as support they provide themselves, because they have promised to pay it back. Depending on your son‘s own income and financial resources, including loans taken out in their name, it might be possible that your son provides more than half his own support. Or, if your son has definitively moved out from your house and has established a permanent residence on his own 2 feet somewhere else, it might be that he is no longer considered to “live with you“ with college being a temporary absence. Those would be two ways in which your son could honestly answer “no, I can’t be claimed as a dependent by someone else.“ But if you or your son are audited (by the IRS or the financial aid bureaucracy), you will have to prove it.
