Could you trust your son to take the pill daily without handing them over to your ex? My son who is now 20, took ADD meds for years and at 12 he would take him meds daily with a little reminder from me. Sounds like your son is more responsible than his dad is to control his medicine.
Horrible that you cannot trust your ex with your son's care.
But I do agree also that you need to fall this fool out on his inappropriate parenting and make is know that you will not tolerate this ever again.