Insurers will want to know, most importantly, who owns the car. They will also want to know in whose name is it registered. Although the issue of registration is not their main concern (they are primarily interested in who the owner is) if the two are different they will want to know why. Your answer may determine whether they offer you cover at all.
If you falsely declare that the car is yours and take a policy out in your name with your son as a named driver, your son could find himself driving uninsured. There have been cases in the recent past where Insurers have determined that the policyholder is not the owner as declared. Accordingly he had no business insuring the vehicle (since, generally, you cannot insure something you do not own) and has furthermore made a false stament of material fact in obtaining the policy. They then retrospectively cancelled the policy, leaving the driver uninsured.
Of course, you may get away with this if your son is not unfortunate enough to be involved in an accident. However, if he is, rigorous enquiries may well be made by your insurers to determine the ownership of the car. Insurers have become very wary of father and son ploy you are thinking of working in an attempt to pay reduced premiums.
My advice would be for your son to take out his own policy for his own car as he ought to. It will cost him more initially, but he will earn his own no claims bonus (reducing his premium considerably after a few years) and he will not fall foul of the law. Bear in mind that if he is convicted of driving with no insurance he will receive a minimum of six penalty points and if this occurs within two years of passing his test he will revert to a provisional licence.