Do not send the father's 1040X with your return. The IRS will not hold up your return and will process it normally and issue you a full refund. This happens often, and you are handling it correctly by claiming the child and mailing the return in. The father claiming the child is only preventing you from e-filing. His actions will not prevent the IRS from issuing you your full refund. No explanation is necessary and including one may cause a delay.
Including the 1040X may cause a delay. The IRS will expect one of you to amend their return. If the other parent does not do this, the IRS will follow up. It is important that the father amends his return and pay any additional tax owed before 04/18/2017. If he waits, he will be subject to interest and penalties.
