seriously, how can you blame the police for your son not remembering he was driving. Of course they don't look at every single picture unless there is need to - most people who receive NIP's would know who was driving, or could at least work it out from the location/time.
I am not a law person, but common sense tells me that as the NIP WAS delivered within the 14 days, it can be counted - simply not the police's fault that your wife and son couldn't remember!
You WILL NOT get away with this. Is he being offered a fixed penalty, or being taken to court?