IME, let it be. Talk to your son if it warrants it, let it be a natural and easy and ongoing conversation between you. Don't mess with your ex. You can't control this, and you certainly can't convince him of his errors here. If I recall, he has a lot of narcissistic tendencies, so I'd naturally let his visitation time slow-fade, and keep your son away from his alcoholism and craziness.
There's a real silver lining to this cloud, and it's that with reduced visitation with your STBXAH your son isn't being exposed to the kinds of insanity that led you to divorce in the first place.