NAL but as far as I know if you have a certified copy of your have primary custody and residency you can and should call the police because he is in contempt of court and the police have to enforce that court order. As primary, unless it is otherwise stated with a visitation schedule you have full control over visitation scheduling as well.

Also take this certified copy of the order to the new school or fax it as he legally cannot keep this child in school as he does not have custody. Explain the situation to the school, the school cannot keep your son enrolled in a situation like this. Have your court order enforced and have it enforced at the school. The police have extra incentive to give your son back to you if he is no longer enrolled and attending school.

After this or even before you need to go down and file an emergency petition on the basis that your ex is violating your court order and jeopardizing your son's safety. This is major disruption to his daily life and therefore his well being. With an emergency petiton the judge should hear your case same day and may make a temporary ruling the same day you file.

If you do not have a court order, or any order on record for custody. File the emergency petition immediately. Without an order the judge should still look at your petition the same day and may make a temporary order based on what is just going on.

With this type of petition you have to show that the ex doing this is dangerous to your son's well being, you have to explain why and give examples. Explain how it is in your son's best interest to have him returned immediately. Make it only about your son and in what ways being with your ex in a situation like this is harmful. If there is anything your ex does now or if he has recent history of doing anything that may harm your son include that as well e.g. drug abuse, alcohol abuse, emotional abuse, domestic violence, arrest history that is relevant, turning the child against you and so on.

Also call CPS, inform them of the situation and that you absolutely need a well check on your child. You are concerned for his safety especially emotional health as this is very disruptive to him. Again explain anything else that maybe harmful to your son. If you have an order in place inform CPS as well.

After this is all settled get a court order that enforces every little aspect of the child and visitation down to the exact times of visitation so it is easier to enforce in the future. Source-my sister just went through the same exact thing(like the same exact thing, for real)and she did all this and got her son back almost immediately.

Try not to worry right now about future custody, first concentrate on the immediate problem at hand. Future custody most likely will not be sorted out for months. At this point you should have a lawyer to help now and with custody in the future. With a lawyer you will have all the help you need. If you can't afford a lawyer there should be an application at the court house to have one appointed for you by the state. If that is not an option look for lawyers who work for the less fortunate in your area and the bar association.

That is all I got, not sure if it will help but I hope it does. I am so sorry you are dealing with this OP and I wish the best of luck to you and your son.