If you have been served, the first thing you should do is retain the services of an experienced attorney.  At a minimum, you should consult with an attorney and let the attorney review the documents served upon you.  The fact that you have been served does not mean that you have to take your son back to his father.  You should file an Answer within 30 days from the date on which you were served.  You may want to include a "Counterclaim" in your response.  The court will make its decision as to the issue of custody.  In doing so, the court will determine what it believes to be the best interest of your son.  In some cases, a Guardian Ad Litem is appointed to investigate what is best for the child.  The Guardian will submit his/her findings to the court upon completion of the investigation.  But, Guardian Ad Litems are not appointed in all cases.  Contact an attorney and request a consultation.