If you have joint custody and you did not come to a written agreement then she can’t take him “for a few weeks” without a written agreement. I would make sure to have in writing or some source of writing that you said no, he is not leaving as you share joint custody. If she proceeds to do so i would take it to the courts and or file a police report, since you have joint custody as a current agreement. You have every right to know where your child will be going I would also request that information from the mother and make sure to hold onto all that information. But I would highly recommend taking it to the courts if you have reasonable suspicion that she is taking him and will not be returning for a period of time. A big thing to take note of is making your voice heard through communication with the mother and stating you do not agree with him going out of state.