I would have a bit of a family meeting and see if you all can come up with some choices in relating to your youngest rather than the switch of his parents doing everything for him (on) or throwing him out (off).

My husband was the oldest of four brothers and I know he helped out a couple of the others when they were in/got out of college; do your other children have any sort of relationship with their brother and/or might they be willing to help out in some way? Now my three stepsons help each other out too; baby sitting the grandkids (oldest helps 2nd oldest and his sister-in-law with that) and they all have a single storage unit they share, etc. The youngest and his wife live in New York City and so the middle brother kept his sister-in-law's car in their driveway (and got to use it when one of their cars broke) for them since they did not want a car in that City. Even if the siblings live too far away, perhaps they left behind friends and connections that the youngest knows or could use?

The youngest needs to be making friends and connections of his own for when he is out of school, can he car pool with anyone when his car is in the shop, call a friend to drop him off to pick it up at the shop, etc.? His car and getting to and from his "work" (school) should be his responsibility entirely.

I'm surprised that, if he is not managing his own life, you give him so much allowance (what does he spend it on?). Has he been taught/learned how to manage money? I don't mind the deal of paying for everything if he goes to school but if he's living in your house he still has to be a citizen of that house and interact with its other residents and account for any monies -- I imagine how much and on what your husband spends his cash and he, what you spend is known to each other? Your son is part of the household and needs to understand and act a part of it. Buying gas for the car, weed, and cell phone coverage are not all there is to being responsible for one's self as part of a larger household. He's not on his own yet!