car insurers not dealing with a claim

Can anyone help regarding a car insurance firm reserving its right not to deal with a claim?
My son was involved in a collision at the end of January. This happened five days after he passed his test. He drove into the back of a Parcel Force delivery van. There was no obvious damage to the van although his car had some damage. He didn't tell his own insurance company he was involved in the collision.
Six weeks later he received a leter from a firm of solicitiors saying the other driver could be seeking settlement for whiplash injuries (although he had not been seen by a medical expert at that point).
My son's insurance company have written to him saying failure to notify them is a serious breach of his policy conditions & consequently they must reserve their right not to deal with the claim, pending the immediate completion & return of an enclosed form together with a satisfactory written explanation for the delay.
So, how can he make things better & avoid making them worse? What sort of explanation is satisfactory? He is 18 years old & has no prior experience of car insurance. Will he be condemned if he pleads ignorance, eg saying he hadn't read the policy & so didn't realise he should have informed them?
The prospect of having to meet a claim for injuries without the insurers is extremely scary. Not only for my son, who is still at school, but for me too!
Sorry for the long post but we need the expereience of others &/or an insider's understanding of how such a situation is dealt with.

Thanks.

He needs to get his forms in to the insurers asap and cross his fingers. Odds on they will pay as the delay will not have put them in a worse position that they would have otherwise been in.

Shame your son's first letter to the insurer informing them about this had not been lost in the post ;-)

WA's coments are well worded and precise.

Good that we have aliases on here.

They will deal as long as he sends the claim form in asap.

I've been caught out like this once too often on my public liability insurance (I'm a farmer and I have several footpaths running across my land). If anyone knocks on my door and makes any sort of complaint at all (caught coat on barbed wire, slipped in sheep poo, your dog barked at me) I report it to my insurance company now. They're probably heartily sick of me, but at least two of the three examples I mentioned above have formed the basis for claims against me - so I feel justifide, in having paid my premium in dumping it in their lap.

He'll likely be OK on this one as WA says, but needs to learn the lesson in this litigous world we now inhabit.

Thanks for your comments people.
The form has been completed and a letter of explanation sent off.

F R - it sounds like hell for the footpath walkers (barking dog scaring them so they fall in sheep poo & then get their coat caught on barbed wire)!!! :)
I don't suppose they query why there's sheep poo there!

You're right about my son learning the lesson too.
Cheers.