You're in a very tough position, but for the safety of ALL (him, yourself, and anyone else on the road), you should tell the doctor immediately. If it can happen while parking a car, it can happen while driving down any road and it doesn't have to be (and likely won't be) in the next state. He will not be permitted to drive for a while to ensure medicine works. It's never easy to give up the independence of driving, but neither of you will never forgive yourself if he has an accident and hurts or kills one of you or someone else.

I would try talking with him again about the potential harm/damage that could come from him ignoring these. His health is a risk as well...not knowing what's causing them.

Good luck to you and him.