Detrimental reliance has nothing to do with this. Obviously, you are in enough contact with him for him to reply to you "obviously, you can do what you want," then why not send another email that says, this is what the dentist suggests, what do you think? So then he can say "obviously, you can do what you want." THEN you have a detrimental reliance issue. It means, that you relied to your detriment that something was going to happen. For example, he said he was going to do something (say pay a bill), and didn't. Now your credit is shot because of the nonpayment. In fact, what does the CO say exactly about medical/dental treatment and expenses? He probably relied to HIS detriment that you were going to follow the CO and not authorize any treatment that he would be financially responsible for that wasn't an emergency.

This is simply a case of you doing what you wanted to do and wanting him to foot half of the bill. If you were sitting in front of my judge, you'd be paying the bill completely. You'll find nothing in the family code that speaks of sharing medical expenses, because it's not mandated by law. It's ordered on a case by case basis.

If anything, if you have something in writing from the DENTIST'S office stating that the copay for the sealants were zero (which you probably don't have because I can't think of one insurance company that covers it), you MAYBE have a detrimental reliance case against THEM, but even still, someone is responsible for veriifying the benefits BEFORE treatment is rendered, and that's not the dentist's office responsibility, either.

You screwed up on this one, and it's going to cost you about $800