If it were not for your child suffering, I would also say wait. Wait for him to do something, either to ask for visitation or for the next demand or threat. Then send all your letters. Once you have his responses, at that point, file your changes.
However, the concern has got to be the child and you need to do whatever you can to help him. You can find threatment that does not require his signiture but until things are sorted with the ex, you will have to pay for the treatment.
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Psst... I do not work for Macdonald’s or Burger King, and even if I did, I would not tell you. For sloppy bread, tired tomatoes, frozen onions, watered down mayonnaise and imitation meat, please find a drive through window with a person who openly admits they make that stuff.
My advice above is equal to the advice they would give if you asked that person a legal question.