No one answer- can be location specific within your state, depends on engine size, whether he is added to your policy (cheapest) or on his own, if he has had driver's ed, and his grades (good student discount)
You REALLY need to sit down with your insurance agent.
Do NOT get the lowest amount your state requires. If he hurts someone, damages their property, and it exceeds your policy limits, since he is a minor, YOU may be left on the hook for the difference.
What we have here is... failure- to communicate.