It sounds like you claimed your stepson as a dependent on your tax return in a situation in which the father has the legal right to claim him in the divorce decree with your wife. In this case, the father can (and likely, will) claim your stepson. Only one taxpayer can claim a child as a dependent. See this article which addresses a separate legal document stipulating the parent who may claim the child.
Since it appears that you and your wife were not entitled to claim your stepson, you should consider filing an amended return to remove your stepson from your return.
I hope this is helpful.
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