If your ex doesn't pay his part of the medical expenses I would go back to court to request it instead of going against a child custody court order which the IRS won't enforce but which the issuing Judge can in other ways. Consider if there are things in this order that work in your favor. If your ex takes you back to court because you broke the agreement, you're the one who will be on record as breaking the agreement so you should prepare to have all those issues up for grab again. If you take him to court for what he hasn't paid, then he'll be the one on record as breaking the agreement.
What I would do is write an email to your ex about unpaid medical expenses stating the amount owed and requesting a date that he will be able to pay you buy and attach all receipts which is important because he doesn't have to pay you if you don't provide proof of payment. That way you have electronic documentation that he was sent this information. If he does not response send your own response in respectful language stating that you are still waiting for a response. This is not only documentation of the debt and your providing the information needed for it, but it also documents that you're being respectful and cooperative in trying to claim it. Judges care about that because ex-spouse/lover/partner drama is childish when a child is in the center of it and parents are expected to be professional and adult in their co-parenting.
