Further to Chris's answer my 24 year old daughter who has had fully comp insurance in her own name on her own car since age 17 was recently added as a named driver on to her boyfriends insurance who at age 24 and passed his test 6 months ago it reduced his premium by over �200 goodness knows why but I would suggest making enquries as to adding yourself as a named driver on your sons insurance.Hopefully you will get more info from a couple on here who are in the car insurance industry.