It is absolutely obligatory to inform your insurance company of any motoring convictions as soon as you learn of them. I have no doubt whatsoever that it is in the small print of your son's policy. They have the right to adjust the premium accordinly as a result of a change in your risk profile. Usually a single offence unless especially serious (or a new driver) will have no effect but they still need to be told. Subsequent offences are likely to add to the cost of the premium.
Not having informed them means they can refuse the claim on the basis of non-disclosure of material facts. All you can do since they clearly now know of it is plead ignorance and hope they don't do so.
I've had two speeding convictions both on a motorway, one in 2005 and one in 2008. I reported both to my insurers straight away. The former had no effect at all, the second cost me an extra £20 or so I think. It also cost my wife an extra £10 as I'm a named driver on her policy. In dicussions with friends and colleagues after the event I was absolutely astounded to discover a large percentage had no concept that they were meant to inform their insurance company of convictions.