My son arrested for auto theft but it is my car?

If this is simply a matter of either (a) the police thought he stole the car from you because they did not know that you let him borrow it, or (b) the police thought he stole the car from someone else because they did not know you bought it, then you go to them and you show them proof that it is your car and tell them that you let him use it. If there is anything more to this, then you hire a lawyer to deal with it.