You won't be able to efile, because your son's SSN is on another return.
In order for you to claim the child, you would have to print your return and mail it, along with an explanation that you are the custodial parent and are entitled to claim the child. The IRS will then open both tax returns and make the determination as to who can claim the exemption. The parent who can't claim the dependent will have the exemption disallowed, and will have to pay back the tax, with interest and possibly penalties.
Please see this link for more information.
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