You need to check that he was not covered by either policy, but from what you've said that is a faint hope. Other than that, your best bet is probably to own up as an honest mistake.
There should be a duty solicitor at court who will advise, but you might want to get advice in advance.
From memory, six points and a fine is a likely punishment.
Why was he driving? You are lucky that you're not in court as I think it is an offence to allow an uninsured driver to use your vehicle.
Take me to your lizard...