it says on the front of your log book that the regictered keeper is not neccessarly the actual owner so it is possible but i'm not actually sure how you would go about this, maby write up some sort of agreement with both signatures, try ringing dvla they should know, sorry i couldnt be of more help but i know it is possible
There is no need in law to do any more than you have. As stated above, the registered keeper is the person who normally has possession, not the owner. If you feel that you need to have a written agreement with your son, that is a matter for you and the DVLA will not be interested.
Many thanks for your replies - have been thinking about this a bit more - my son has 6 years no claim discount - if I insure the car with him as a named driver, how long can he keep the discount without owning a car?
I think you will find the insurance company will require the vehicle to be registered in the name of the first insured. He may as well insure it himself especially if he has a good no claims bonus.