You need to get in with a local divorce attorney ASAP. There may be a cutoff for when your son can be reinstated. Take your divorce judgment and any other papers like a joint parenting agreement or any orders entered by the court regarding these matters after the divorce. We cannot really answer you on here because we do not have all the facts. It sure sounds like a reasonable solution that you propose.
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You need to take your judgment for dissolution of marriage and any other relevant paperwork to an attorney to discuss this in more detail. Generally speaking, support terminates upon the acquisition of a baccalaureate degree. However recent changes in the law permit parents to maintain their children as covered insureds until the age of 26.