If you knew he didn't come round what stopped you from finding him?

Were you stopped at ticket barriers or as you were trying to exit the station? You have all ready mentioned that you didn't have an answer when asked "how would your fare have been paid?" The role of the RPO was to ascertain the facts of the situation and he obviously had sufficient infomation to report the matter, many people on a daily basis frauduently travel over distance on season tickets, your best sitting and waiting until the TOC writes and explains why exactly you were reported on that day.

Student id still doesn't prove that your son was in possesion of a valid railcard, not ALL students have railcards as some councils will offer assisted travel to students, I don't think the answers to your questions are defensive but most probably the answers you are least likely to want to hear.

How do you know that it was an attempt to steal details, perhaps someone gave your details as false to avoid any future contact.

You need to put yourself in the shoes of the person who issued the PFN, how were they to know if your son had a railcard or not? Most will but there is always people willing to abuse the system and this is why discretion cannot be shown in cases where the railcard cannot be produced at point of being asked, Also if buying the railcard at the ticket office when using the relevant application/leaflet you will find the t&c's in the back of the leaflet and this is given back to the customer when purchase is complete of the relevant railcard.

The T&C's of your card that you signed or ticked for clearly state that your discounted ticket is only valid upon producing the relevant railcard, you clearly could not produce the supporting document so you should count yourself lucky its only £35 and nothing more.