Well since you gave permission, he has done nothing wrong.
I don’t wish to come across as disrespectful, yet I don’t’ see the logic to your response.
This jaybird of roommate skipped out of town, left the state, took the OP’s vehicle and other stuff, no forwarding address, has not returned the vehicle nor given the owner to know where he can find and recover it, and you are saying that the guy did nothing wrong?
True, it is given that the guy was authorized to use the vehicle, but we don’t know the terms, conditions, purpose, duration and geographical scope of any such permitted usage, or whether or not consideration passed.
It would be reasonable for the OP to interpret your response as meaning that once an owner of personal property grants possessory use of it to another, that such possession can never evolve into an act of criminal appropriation. And that isn’t sound law. An embezzler is presupposed to have a possessory right.
(And why couldn’t this be an act of embezzlement as well as violation of the Dyer Act - 18 USCS § 2311 et seq.)
Plus your post seems to overlook the right of the owner to withdraw consent to the use.
If the OP told some donut receptacle hiding in a Kentucky cop’s garb that he expressly agreed that the guy could take his car across the Ohio River to some unknown address and keep it indefinitely, that is one thing and your response would be right on.
But we don’t have that one thing and it is absurd to think that this much distraught and seemingly little educated OP will confess to it.