Drifting a smidge OT, I appreciate, but here's the relevent ACPO guidelines, and bandings, for how to prosecute motorists exceeding the speed limit:
View attachment 45483
Yes, he may well have been able to avail of a speed awareness course instead of a fine/ban/points, but that's only an option if the registered owner of the vehicle replies to the original notice of intended prosecution within 30 days. If there's no reply received - in your son's case, because DVLA held an old address and the police didn't hear back from him within 30 days - then it automatically moves on to a summons.
I think you might have been lucky!
But as per the ACPO guidance, there's an element of police discretion permissable - although travelling over the legal speed limit, given the road conditions and time of day, your danger to yourself and others would likely have been less than someone driving at 40mph along a busy shopping street on a Saturday afternoon...
The main thing is that you learned a lesson and never did it again, so from a policing perspective, that's a proportionate response to a detected offence which best serves the public interest.
I'm not an expert, by the way - just someone who's had reason to do a little research on the topic lately.
View attachment 45484