There are a lot of good responses on this thread, and there is great wisdom on this forum in general. Welcome.
From my life I can testify that high intelligence can compensate for many social difficulties. Especially if left to their own devices - such as with a parent that is an alcoholic or otherwise distracted - ASD kids will create proprietary methods of understanding the world around them, including strange language and one-off explanations for how things work. I would call it a highly proprietary user's manual for their environment, and others are unlikely to understand it's contents or logic. Because of this, it is terribly hard to correct an ASD because you don't speak their language and can come across as tyrannical or threatening instead of a mentor or translator.
That being said, intervention now is important to prevent him from collapsing inward until he is unable to cope with life and ends up a lifelong burden to himself and his family. It will take time and lots of energy, but I'm confident you can help him forge his best life. I can tell you are strongly emotionalizing your role in his life, and that's great. He needs a competent advocate.
Keeping in mind that autism isn't definable like football or the flu, I'm going to throw out some things applicable to me that may or may not be helpful to your situation.
I hate surprises. When I'm being taught something I need to know what you are going to do. If you were teaching me how to drive, for example, and you may reach over and correct the steering wheel I need to know this in advance and your justification for it. Once the ground rules are set I won't be so triggered or surprised.
On driving; it took me years and years to understand that "the rules" aren't directives for proper operation of a motor vehicle, but instead are imperfect guidelines for the movement of people and societies' way of assigning liability for wrecks. It is tough for me to tolerate rules that aren't concrete or followed by everyone. Making things worse are police, who are supposed to enforce "the rules" but are terribly inconsistent in everything they say and do.
Further, I have a very good relationship with my mind that doesn't always extend to my body. When working with my hands, for example, they don't always do what my mind wants to see happen and it is frustrating, even triggering when repetitive. It doesn't matter if I'm cutting a board or pressing the accelerator of a car - if what I want to happen in my mind doesn't happen in reality I can get pissed and have to step away for a bit. This may help explain some of the problems your stepson has driving. However, it is not uncommon for autism to accompany other developmental disabilities and he may have lots of trouble learning to adapt to driving, amongst many other things. If he learns to drive independently and you are at all concerned, put everything in his name or form an LLC to protect you if the worst happens.
Oh! For the love of God teach him how to cook, plan meals, and grocery shop. Nobody taught me any of this. When I moved into my first place it had a fridge, which was empty. Instinctually, I ran off to the grocery store and spent over a hundred dollars on ketchup, pickles, mayonnaise, and a bunch of other things that all fridges have in them. Only after taking some time to find the right place for every single condiment did I notice that I hadn't bought any food.
Not only with cooking and meal prep, you want to form his routines in a way that he can continue them unaided. If everyone could leave the house for a few days and you know he'd bath and be fed then he can do the same in his own apartment.
