I never advised my carrier whenever my kids got a license. When they found out (usually after an accident) they then raised my premium.
As I said earlier:
As an additional point, if your son drives someone elses car, he may not be covered either unless the named insured (parent or spouse) allows them to drive on an occasional basis. If your son’s friend allows him to drive an auto under his parents’ policy, their auto insurer is within their rights to deny coverage (reasonable belief exclusion).