A bit late, I know cleggie (only saw this question today).
Perhaps one way to demonstrate this to him is to suggest that he could go and buy a policy for a �500 banger. He could then scrap or sell the banger, keep the policy and go on to drive his mate�s Ferrari. Clearly ridiculous. He sounds as if he knows he is right whatever you say, so it may be wasted breath.
More importantly there is another warning you (or rather Mum) needs to heed.
If he is caught driving with no insurance he will be prosecuted, but so will mum. It is an offence for the Registered Keeper of a vehicle to allow it to be driven uninsured. The penalty is the same as for driving uninsured (a fine plus a minimum of six points) and the excuse that �he told me he was insured and I believed him/wasn�t quite sure but I still let him drive...� etc will cut no ice.