Your friend may well have difficulty insuring his son�s car. This is particularly so if he wants anything other than Third Party only insurance (which few companies offer these days anyway).
The insurers will need to be satisfied that he has a �financial interest� in the vehicle (something which he strictly does not have). This is because, in the event of it being stolen or otherwise written off, the insurers will be liable to pay your friend (their policyholder) for its loss and not the owner. Taken to an extreme, I could take out insurance on my neighbour�s Ferrari, arrange for it to be stolen, and then claim for its loss on the policy I have taken out. In principle you cannot generally insure something that is not yours.
The insurers will also want to know why the owner and registered keeper of the vehicle are not one and the same and may be reluctant to issue a policy when they find out the circumstances. I am not saying your friend will be unable to insure his son�s car but, despite what has been said, it is not straightforward.