I would suggest that you go back through his information to make sure that you answered everything correctly. Answering one question incorrectly will disqualify him from being your dependent. As Fanfare has mentioned, it could also be that your son no longer qualifies to be your dependent.
Generally, if they are under age 24, full-time students, live with you, and don't pay more than half of their own support, you can claim them as dependents.
If they had income, they can file their own returns, but they must state in their return that someone else is claiming them as dependents.
See the TurboTax help articles below for more information:
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