If your son was claimed by his father and has already Efiled, you will not be able to Efile your return and claim your son.
You can still claim him if you are entitled to, but you will need to mail in your return.
If his dad was not entitled to claim him, he can Amend his Return after he receives his Refund.
The IRS will eventually contact both of you to determine who had the right to claim him, and the other party may have to return part of the refund they received by claiming him.
Here's more info:
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