Examine the language of your order carefully. If you share joint legal custody either parent is authorised to act.
Some orders limit the ability to act without consultation or agreement.
If yours limits the ability to act alone he may have violated the order. On the practical side you should communicate with the doctor asasp and learn what you need to know about the diagnosis and treatment plan. If you are not in agreement discuss same with the ex. If you cannot reach a resolve you may have to file an application with the court for an order to correct the problem.