He needed to be informed within 14 days (Notice of Intended Prosecution) that he could/would be prosecuted...this has been complied with, with the photo.
Normally the rest of it needs to be completed within 6 months, because if your son wished to go to court instead of pay a conditional ticket, then the information to be laid before the Magistrates by the prosecuting authority, for them to consider issuing a summons, needs to be completed within 6 months. The summons itself can come out after the 6 months, as long as the 'information' has been laid.
After being flashed son should have within 14 days (less date of offence) paperwork containing a written note to the effect that he is to be Prosecuted (Notice of Intended Prosecution) together, if he is the Reg keeper of the vehicle, a form under section 172 Road Traffic act 1988 requesting that he name the driver of the vehicle WHICH HAS TO BE COMPLETED AND RETURNED with 28 days. Offence not to do so.
If there was no information that he accepts the offence and he was the driver and to discharge proceedings by paying £60 and forwarding Licence for the 3 points (known as a Conditional Offer) then it may well be this will follow once 172 form returned naming himself as driver,
His speed was outside the guide lines for a Conditional Offer then the case will be going to Court and may be why they sent a photo?, 3 - 4 months approx for summons to follow.
If there as 9 points still active on his Driving Licence and whilst his speed was well within the guide lines for Conditional Offer, a further 3 points from this offence would put him up for the totting rules ( 12 in 3 years) requiring a Court hearing and possible disqualification.