Social Security & Medicare

Eventually an audit, social workers, or someone will report it or a query will flag the discrepancy, and the state will act. Since you both are claiming the child resides with each of you, you will have to prove that your custody claim is the legal and valid one, not the stepson. Questions may arise about what did you know that might make you look like an accomplice. Maybe it can be resolved simply by just providing your custody orders, but there's a possibility that you might need a lawyer to protect yourself if they press charges against your stepson.

Consider this too, if he's already claiming custody, then is it likely that he is also claiming the tax deduction too? Now the feds would be involved and asking who really has the valid claim.

Your primary goal should be to protect yourself so you're able to give the boy a safe and loving home.