Request from your driving instructor the document your son signed advising as to what liability he would accept should he damage the vehicle while out on a lesson. If there is no contract, there is no liability on your son to pay. It could be argued that the instructor has been negligent in his tuition in not preventing your son from hitting the kerb (if he has dual controls). I would be advising that it is the instructors responsibility and part and parcel of the job. You also have the fact that as your son is 17, he cannot lawfully enter into a contract.