I can only recommend that you insure him on his insurance policy and add yourself as an additional driver as he will be driving the car the most.
It will cost a huge amount for the first two - three years but totally worth it in the end. I was on my mothers insurance for 3 years, then decided to get my own insurance and ended up paying lots just to get my own policy.
Also as your son is young he stands quite a high chance of having an accident in his first 1-2 years of driving and it would be better for you that he's not on your policy when/if this happens.
Avoid the bonus accelarator policies as these NCB are not transferable between insurance companies and you will want to switch insurance on a year basis to make the maximum saving you can.
Also look at the tips on the moneysupermarket question pages to help reduce the cost of insurance. Eg If the car has no alarm look at getting an alarm.
No matter what you do expect a high price even on your policy, I would say upwards of £500. Most importantly shop around.