I know insurance companies are trying to cut down on any extras offered, so that they can offer the lowest premiums � but I would check the whole policy document wording (small print), your son may be covered.
When I started to drive, I had a guy with a red flag walking in front of me (not sure why), but it was absolutely standard for all comprehensive policies to include the policy holder driving another car (on the proviso it was not owned by the policy holder and not on hire).
You were only covered third party, but you were covered.
Looking at my current policy, this is still so, for me at least.