You cant tell by their words if they will change. Words are cheap.
Actions matter. Next time block out what he is saying, promising, swearing he will do....and go strictly by what he IS doing. Doing. Not saying.
If he is going to work each day, listening to societys rules, paying his own bills etc. he is being a responsible adult.
Most of the adult kids who bring us to this forum talk like gifted salesmen, but dont perform. Start thinking of the performance as what tells you how they think and feel and what thet value and where they are in life. Most responsible adults have no desire to live with parents.
It is easy to promise to work, pay rent, stop drugs etc. It means nothing. How they are acting says more than tjeir self serving promises.
Heavens, your son can either pay his own car bills, insurance, repairs and gas or he can walk or rely on friends. I never gave my kids free cars or free use of any. Its not a necessity
I am sorry that your son let you down. Try to do something nice for yourself tonight and put him out of your mind if you can. Angsting over him wont help you or him.
Love and light!