If this is a Rhode Island case you may file a motion seeking to adjudge him in contempt. However, if the order did not require your ex to pay for health insurance beyond the boy's 18th birthday you will probably not be able to get him to pay for it now. As always, without a complete consultation I can merely offer guidance, not advice. You should have a complete discussion with a qualified attorney in order to obtain the best advice.
Feel free to contact me directly to discuss this matter.