If you file your return claiming your son as a dependent and don't provide his social security number (SSN) on your return, the IRS will not allow you to claim him as a dependent.
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Why would you not have your child's SSN? If you lost the SS card then you might need to get a repayment from the SSA. That might require you to visit a local SSA office with the child's birth certificate. Call the SSA for guidance.