How would I know if this is considered cosmetic and would I be responsible for it if he goes against my wishes of having it done now instead of later?

My ex and I have joint custody of our son (14) who has been living with his dad now for about 1 ½ years. We are supposed to communicate and jointly decide medical, dental, etc. however he is not doing that until he has already had it done or at the office having it done. He recently called saying he was at the orthodontist getting our son braces (mold, etc.), said he is doing a payment plan for $xx amount. We didn’t discuss this and now he is just doing what he wants with my money (I cannot afford right now). I carry our son on my insurance which is currently changing so I was able to stop the procedure when I called and talked to the office (once I found out where it was). My son doesn’t like his smile so he wants braces but he doesn’t need them. I am responsible for 62% of everything and just cannot afford that large bill right now. How do I proceed?