What will happen if my 18 year old son stole a car with no license and he puts a different plate on it?

"picked up the car" ??? As in they were sent to get a car they thought belonged to the one who sent them? As in they saw a car and they took it? It's a hard sell he didn't know the car was stolen if it had no plate and they acquired a plate for the car, especially if that acquisition did not involve a tag agency. He needs counsel.