Isn't parenting fun?
Personally I think you are being too easy on him. He lapsed twice in a very short time and you relented and are letting him both stay and use your car. I think the car at least should be off limits for him for a long time. And one more infraction would put him out the door. He needs to know that you mean what you say and he has reached the bottom line.
As for you doing the same thing when you were young, it was a different time then with very different drug strengths. What you smoked would not compare with what he has put into his body.