Would I be saving money if I bought a car and did the liability only insurance policy?

My son was on our policy, he was named as a driver for all three vehicles. He went and got his own policy, full coverage, now he is only listed to drive his truck (Ford F150). That cut his insurance by more than half. My wife and I are still on my policy, 2 vehicles with full coverage on each, it also cut my insurance by more than half. When you get your own policy go back to your parents ins co and show them your proof of ins and tell them you want off of their policy. The only hold up I can see is that you are only 17. When you turn 18 the laws will change, I know it's only a number.