It sounds like this runs deeper than ADHD denial. For this to happen in the first place means that mutual respect and active communication are lacking between you and your ex. This means you don’t have a unified plan of parenting, and of course that means that you two can stray far from each other in your parenting decisions.
As a child of divorced parents, let me say there’s no benefit to raging a battle against your ex or trying to control what he does when he has the kid(s). It only hurts the kids. Try and find a way of addressing this that doesn’t feel like an attack or an order, because once defensiveness sets in, winning the battle is all you two will be able to focus on. Assuming your ex doesn’t have abusive tendencies, or the strong need to control others, a good therapist might be able to help facilitate a productive conversation. However, this won’t work with anyone that has narcissistic tendencies or strong ideological leanings.
If it isn’t possible to create mutual respect and effective communication in raising your child, you might have to just let it be and explain as best you can to your child that he must adjust to different rules between parents. In that situation, you can also share the scientific information with your child. Let him know all about his condition, why medication helps, and the risks of not being on meds. We tend to think kids won’t understand, but we’re so often wrong.
Your son may not show appreciation for it, but he’ll be paying closer attention to the difference between being on the meds and being off if you talk about it in a non-confrontational way enough. He may very well gain an appreciation for its benefits.