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I will try to best answer your question.
I am going to assume that mother is having a difficult time controlling your son. I'm also going to assume that she did not discuss this with you at all in advance and you had no idea this level of problem was occurring.
If there is a custody order in place, technically speaking, mother is in contempt by failing to provide for your son or filing for modification of the existing order before sending him to you to live. If you are not able to care for or provide for your son you could send him back to mother.
If you want to try to keep your son, you would need to file petition for modification of the custody order so that you are the primary custodian and then filed to terminate any support action that you are paying and then file a complaint for support against mother to pay you. If she wants you to be the primary custodian she would need to pay you child support to assist you with doing so.
I hope that this information is helpful and I wish you the best of luck trying to resolve this difficult situation.