As flip-plop and others suggest, your son should plead guilty. The onus is upon him to be absolutely certain he is insured before driving.
Your son will not have access to the duty solicitor. Defendants can only see them if they are otherwise unrepresented and are charged with an offence which is imprisonable. For similar reasons legal aid will not be available.
However, as has been said, there is no need. He should plead guilty, and explain the circumstances to mitigate the offence. This is unlikely to cut much ice, however, and he can expect a fine of around �200, plus a contribution towards prosecution costs of around �40, plus a �15 �Victim Surcharge�, and six penalty points. Hopefully he has held a full licence for more than two years. If not his licence will be revoked, he will revert to provisional status and will have to take his test again.