Per MY dentist and my son's dentist, sealants are NOT medically necessary. Yeah, they prefer them, yes, it makes them money and if I were your ex, you would be footing 100% of that bill.

What was your ex's response when you approached him about the dental sealants BEFORE you had the treatment done? I'm assuming you have joint legal custody and have to make that information available to him. Did he agree to the treatment PRIOR to the treatment being performed? If he did, then he's on the hook. If you decided to do it first and tell him later, then you're on the hook.