Quite simply, in the worst case scenario, if the police officer is then familiar with these devices, criminal proceedings would probably be opened against your son. That would be like him driving a car at the age of 17 without an ID card, which is not registered or otherwise not allowed to be driven on the road. Or driving a souped-up moped without a licence. Whether he would then be allowed to take the car test at 18, I cannot say. In short: Putting a vehicle into circulation that is not registered for road traffic in combination with "without authorisation" to drive this vehicle = not good!