You would send a written revocation letter to your son's father and anyone he may have been dealing with. The idea is to provide notice to anyone concerned that the form is no longer valid.
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It sounds as though your son is a minor (under 18 years) in which case what you are really trying to do is ensure that only you have sole legal custody (the right to make all decisions for your son). Legal custody of a minor is an issue that must be addressed by a court. You might want to re-post this question under the heading of "family law" or "child custody" and include what has happened that makes you now want to not have his father involved.
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