Consider sending him an email with the appointments that are scheduled and explaining the difficulty in getting the appointments and the harm to the children if he allows their meds to run out or not to be returned for when the children are with you. Ask that he acknowledge the appointments can stand and find out what participation he'd like to have in taking the children to the appointments. Be prepared to explain, if you didn't coordinate the appointments with him or simply scheduled them during his time-sharing time, why you did so.
Maybe he can take them to some of the appointments. If you get nowhere, then consider asking by a written motion filed in the case that the court grant you sole parental responsibility over medical decisions and scheduling appointments for the children. That process may not help you with the immediate summer appointments, if you can't get in quickly to see a judge, but may help the children down the road.
Please note that this response is not intended to create any attorney-client relationship and is only based on the limited facts given. The response might change should additional facts be learned and should not be relied on as legal advice. It is recommended that you consult with an attorney who can properly assess the situation, as well as all pertinent facts, prior to taking any action based on the foregoing statements.
You should absolutely inform him of the appointment dates and times and an expectation that he will either take them to the appointments or allow you to pick up the children for the appointments. If you can do this by email and get his response as quickly as possible. If he refuses to do either, you can file an Emergency Motion with the court requesting the right to take the children to their appointments as scheduled, due to his refusal. You will have to move quickly as hearing time is extremely hard to come by and the court may deem this not to be an emergency.
Sign up to receive a 5-part series of useful information and advice about child custody law.