This post largely reiterates what Redcrx has said, but it's important, so I'll post anyway:
If your son has a car for his sole use (or for which he's the principal driver) he must have an insurance policy in his own name. (It's irrelevant as to whether he actually owns the car or whether he's the registered keeper of it). He can't be covered as an 'additional driver' on your own policy. Attempting to get insurance cover for him, via that route, is called 'fronting' by the insurance companies but the charge which would be read out in court (potentially leading to a lengthy prison sentence) would be 'fraud by false representation'.
As Redcrx has indicated, adding a more experienced driver (such as yourself) onto his OWN policy (as a 'named driver') can result in a significant reduction in the premium.
He should also check out all types of insurance for his car, as 'fully comprehensive' can sometimes be cheaper than 'third party, fire & theft' or just 'third party'.
He should also try to get a car in as low an insurance group as possible:
http://www.parkers.co.uk/insurance/#ig=0
Pass Plus could also help your son reduce his insurance premium:
http://www.direct.gov...iedDrivers/DG_4022426
Chris