You have control of your choices and what you choose to do today. Thinking we can control another is an illusion, so there is no loss of control when they don't do what we want them to do. It took me a long time to learn that.
As the mama of an addict, I would want to know before I took responsibility for a vehicle being in my name and driven by someone active in addiction.
Why not transfer it to his name and then let go of where it goes from there? Or simply tell him he will have to arrange his own transportation. If he can make it to find drugs, he can find a way to go to work...or not.
I know how your heart hurts right now, but if you just focus on you and plan better days ahead for yourself, I think you will see things fall into place.
Hugs