In addition to what Ethel has said I would add that the best suggestion is for him to plead guilty and then put forward what you have said in mitigation to assist him. You could argue "special reasons" for your son not to have his licence endorced. In that case you would have to give evidence to say that you all genuinely thought he was covered, despite what the insurance company say (and in fairness, I have just had a real spat with my grandparents' insurance company about what they say and what the policy actually says).
When he has to be appear in court, you should make sure you are there to give evidence, if necessary. The worst thing he can do is turn up and say "my dad can confirm, but he is not here....".
As for the technicality, forget it. Under s123 Magistrates Courts Act 1980 (I think) the prosecution has a right to amend the summons at any time. The best your son is entitled to is an adjournment, but that is not going to help matters, given that you KNOW it was at night, and not in the morning.