I am 100% in agreement with akrasia.
I'm living the nightmare where I have to always make sure my children have their cellphones, and where I have to tell a child whose age is still in the single digits to never, never, ever get into the car with her father when he's been drinking.
My girlfriend's son was killed by a drunk driver when he was 17. I have zero tolerance for drunk driving. I would do whatever it takes to make sure my child does not ride with a person who has so little concern for other people's lives that he gets behind the wheel after drinking.
My divorce agreement, that my AXH signed, says he will abstain from alcohol while the children are in his care. He drinks every day, whether they are there or not. Compassion for the alcoholic has nothing to do with the fact that your first job is keeping your child safe. Agreements with an alcoholic aren't worth the paper they're written on. They last until s/he gets the urge to have a drink.
I know you're worried about alienating your son. But you have a son who's alive. Make sure he stays alive, so that you can work on the emotional part. If he gets killed riding with your ex when he's drunk, you will never forgive yourself for not having done everything in your might to protect him.