Assuming you have shared legal custody of the child and he does not return your child to you at the conclusion of his custodial time (whether it is because he took the child out of state or an unannounced vacation), you would file for contempt of the custody order. Quite simply, he has violated the Custody Order and you need to enforce it.
If you are unable to locate your child, though, you may need to also call law enforcement to help you track down the father. Keep in mind that law enforcement officers typically won't enforce your custody order (they defer to the courts for that), but they may help you locate your child so you can serve the father with the contempt action. If the father has acted criminally in any way, then the police will charge him appropriately.
This answer is for the purpose of providing general information and should not be construed as legal advice or the basis for an attorney-client relationship. This answer is limited to the the information provided in the question and should not be construed as an exhaustive response. It is recommended that the questioner have a consultation with an attorney for an answer specific to their situation.
The bigger question for you really is what if he takes your child out of state and doesn't return him not whether he takes him out of state to visit family and then brings him back. Your custody order should specifically set forth what notification provisions are required when traveling with your child out of state and/or taking him on vacation. If there are no written notice requirements, then he has not violated anything. If there are, then not notifiying you of his trip out of state would be a violation and you could seek a contempt petition.
However, you need to do a cost-benefit analysis. If he brings your child back, where you really harmed and is it worth the financial and emotional costs of going back to court on a contempt action?
If he fails to return your child, I agree that the first step is contacting law enforcement where you live and where his family lives and he may be staying at with your child. I would then also immediately seek relief with the Family Court via a Contempt Petition.
An attorney-client relationship shall not be formed due to the response to the asked questions. The suggestions made are intended to inform and not advise and are based upon general statements of PA laws and specific events or facts may alter the law. You should contact an experienced PA family law attorney for specific legal advice regarding your issue.
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