You may consider filing a motion to modify visitation, including restricting punishment and permitting your son to call him during visitations. However, unless there is abuse under Maryland law, courts are hesitate to interfere with parental rights. You stated the rules and punishment are different from what he receives at home. However, you didn't state what rules and punishment were offensive to you. Also, keep in mind, the judge may think that you are poisoning your son's mind against his father. This may cause adverse consequences if the father petitions for greater time with your son.
In addition, before filing the motion, you may think about having your son see a mental health professional. His or her findings may support your motion or discourage you from moving forward.
Also, if there is abuse that is recognized under Maryland law, you should call child protective services. However, don't file a false report or call because the father restricts your son from watching TV or eating junk food.
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