Welcome to SR housework.
I don't have experience with an alcoholic child, but I wanted to welcome you here.
I also can't answer your question as to why your son can't just not drive. It seems that many alcoholics have multiple DUI's. I suppose it's because they maybe feel they won't get caught, or maybe it's because being drunk they aren't thinking clearly - I really don't know. What I have found is that it just doesn't benefit me to bang my head against the wall in frustration trying to answer questions about the "why's" of alcoholism. I will never understand.
Do you attend Alanon?