The car needs to go. My son went to school 10 miles away from home, and my husband traveled. I was immobilized in a back brace for 4 months, so we let him keep using it because he "needed it" to get to school.
That was a big mistake. We should have told him he needed to find his way there himself. We were so focused on making exceptions so that we could get him to graduate that we lost focused on the big picture - his life. As it was, we wound up doing an intervention 3 weeks before the end of classes his senior year. No prom- no graduation-no senior week. Just lots of rehab...
He did wind up getting his diploma, and is now in college.
Take the car away - when he asks how he'll get to school, telll him it's his problem. If you can afford inpatient rehab, that would be a good idea. It may not "stick" but the tools he'll learn may come in handy later...it will get him out of the house, and keep him clean until he's 18 and you can send him off on his own.