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first, he should go to the clerk of the court and file a petition for a restraining order for the harassment - save the text messages, of course - if that is issued then contact he has with your son would be a contempt of court and he could be arrested
as to the case of the text messages due to the child support - you can consult with a local attorney about a possible lawsuit for your son's pain and suffering due to his father's remarks - yes, it is possible. you just need to find a lawyer willing to take the matter - which is going to depend greatly on whether a judgement is obtainable - meaning a lawyer is not going to sue someone who does not have money to pay any judgement ordered
you should have a good lawyer assisting in collecting that arreargage - you also can contact your local Child Support Enforcement Agency who will assist in collections - they can attach wages, bank accounts, inheritances, etc