In reply to Rob Exile Ward:
Despite this seeming far to complicated for mere mortals to follow given some if the responses, you're legally obliged to get a ticket at the earliest chance. Everywhere. Has been for a long time.
Why do we have different experiences?
1/ the only way to be sure any excuse is invalid is to have a staffed ticket barrier (staffed in case of a technical fault, the barrier can just be a pemanently in place person) at every station
The cost of doing that means it will never happen.
2/ it's usually not going to be worth delaying the services and potentially endangering staff to have the guard try and get details from everyone who has got on without a ticket, and the alternative is to arrange British transport police to be waiting at the next station which is even worse.
Therefore generally a bit of give and take is allowed and assuming the traveller does not try to avoid paying when asked, or tries to deliberately avoid the people doing the asking, you're almost always OK.
Your son obviously got someone who was having a bad day or whatever and thought he'd follow the rules to the letter. This can occur quite believably due to the immense amount of shit they sometimes have to put up with from people who treat them like the enemy rather than just people doing their jobs sadly.
But that doesn't mean it's a con.
You can get caught speeding and be let off because the officer doesn't feel it's worth (or can't be arsed) taking it further, but you wouldn't say its a con when next time they charge you
Now the Hysteria is past about rail company conspiracies....
As my first post said, you can appeal. Put in some photos and an explanation, offer to pay the fare. Explain that there was no intent to avoid the fare, just an inadequately positioned ticket machine.
You'll very likely hear nothing back about it. The world keeps turning. You've really not gone through some horrible abuse by Mr fatcat who's got it in for you.
IF it goes further then you can get wound up and throw a hiss then