It doesn’t matter what dad will say or do. It is a health issue. This is above him or you.
Keep a calendar with ALL his attacks and his symptoms. You are doing the right thing. You are not doing anything wrong or anything that may hurt you and your case. It is not only your right but your DUTY to take care of the well being of your child. Also keep a calendar every time your child goes to his dad and comes back.
Go an extra mile, if I was you, I would take him to another doctor and if the second doctor diagnoses your son with the same thing, you will have two specialists in the problem at your side, and he cannot complain. This way if you EVER want to fight for full custody, this will look very good for you; by showing the judge that you are a very active mother that loves and cares for her child above anything else and if he fights it will also show the judge that your ex main concern is to pick a fight with you.
I know is frustrating, but will get better.
I would suggest talking to the child's doctor and/or a divorce/family lawyer. Someone who has your son's diagnosis could hurt himself or someone else without meds.
We also have joint legal custody and I have physical custody. My son was having problems with depression and anxiety. He, too, hides these things from his dad. He dad has never seen a meltdown or heard him rant and rave that he wishes he were dead. I document EVERYTHING. His dad is opposed to prescription medication so I was a little nervous that he'd 'freak out'. I inform him of all doctor's visits, pending appointments, etc. I also inform him when he has outbursts, when he talks of wanting to die, and when he talks of wanting to kill himself. E-mail is a great way to inform him...then you have record that he knew about these incidents. If he responds via e-mail then you also have record of how he responds. Jerk knew of the doctor's appointment before we went (he never offered to go or anything like that) so after the appointment I called Jerk to tell him how it went and that he prescribed medication. I cringed...but I was pleasantly surprised by his response. He asked a lot of questions about the medication, side affects, longevity, etc. He made sure my son had the medication when he visited him. I also inform him as things are improving. This helps strengthen my case that the meds are necessary and are working.
Your child's safety (and perhaps the safety of those around him) are at stake here. Do you have any other options? I don't think so. Let us know how he responds.