Everyone has said good stuff here. Just wanted to add - be patient with your son.
I say this because I remember my codie mom - an angry explosive woman married to a habitual alcoholic - was frankly a real bag about it. When I was a child I literally could not see the connections between alcohol and AF's behavior. When I did finally grow old enough and realized the magnitude of the alcohol-induced neglect, my bitter mom called me stupid for "taking so long to see it". She wasn't interested in listening to my perspective; just regularly reinforced how stupid I was. I was already stupid enough for defending AF (whom I believed really did have life so hard that he needed to drink to cope). Now I was doubly stupid for growing up. I could never win with either parent.
Kids don't need to feel like their parents' behavior is their fault, and brain ninja comments like that are simply toxic. From the sounds of it, you know better than my mom than to blame a child for being "too stupid". If anything, this is a reminder of one easy mistake you can avoid. Be the parent that your son can be successful with, and I believe his self-confidence can survive.
I really hope your son will learn to detach from his dad. It sounds like he's struggling with figuring out what he's done wrong to deserve dad's lies and behavior. Keep guiding him out of self-blame. It sounds like you're doing a terrific job so far. You're already doing the right thing in being CONSISTENTLY honest with him.
I wonder if it would help your son to understand that getting into a vehicle with a drunk driver is NEVER okay with ANYONE. Sometimes that just happens to be dad. Phrasing it that way might help take the focus off of dad's behavior and put it back on your son's well-being. It will send a message of what your son can do right instead of focussing on what dad does wrong.
- from an ACoA who frequently watched her AF get out of the vehicle after driving her home and take 10 minutes to crawl up the steps into the house because he was too drunk to walk.