You will need to retain an experienced family law attorney to represent you in this matter. There are complicated custody issues that need to be addressed and you may need to file either a Petition to modify the existing order or a Petition for Special Relief if he seeks to change the circumstances that he previously agreed upon. If he refuses to comply with your existing order, you may also need to file a Petition for Contempt. However, an attorney who is familiar with all of facts and circumstances of your case can give you more specific advice and direction.
My response is based solely on the limited information contained in the question. It is not meant to substitute your attorney's advice.
Yes. Based upon the facts provided, I would file for custody to ensure your son is returned in a timely fashion and your agreement is memorialized and enforceable by the Court. I practice in Berks and am more than happy to assist you. Either way, I believe a consultation with a family law attorney is a prudent decision.
I agree you should discuss the details of your case with an attorney first, but yes it may be in your best interest to file for a modification of the existing agreement.