Dave if the goal was to avoid a criminal charge and putting cost one side how would you proceed if you were in our position?
I would commence a dual-stranded simultaneous strategy. One strand would be to negotiate with the Railway Company's Prosecutor (informing them of the other strand) hoping to reach an agreement to abandon the Prosecution; the other would be to prepare to contest the Prosecution in Court (knowledge of which would immediately trigger the prospects of further costs to both parties).
This is something that could be put in motion by a local solicitor with an established track-record in defence work in a firm which specialises in Criminal Law. A cost of just a few hundred pounds should cover their fees. You will agree a basis of fee payments, and these might range from 'no-win-no-fee' to costs payable if unsuccesful and costs claimed against the Company if successful. (The popular interest on here for an 'Out-of-Court' settlement usually incurs a loss of costs which the desperate passenger usually overlooks at the time of seeking an agreement but does come along as another cost quite rapidly!). I would respectfully suggest that having instructed that local solicitor to act for the overseas son, and managing to take instruction from him directly and providing their proof of identity (as will be required by all solicitors), then a specialist advisor with knowledge of railway law might be able to point them towards the line of reasoning which undermines the charge as presented in the documents received (and leads, either, to the Prosecution being abandoned, or, to a decision in Court in your son's favour - with recovery of all costs fron the Company).
. . . an obvious theoretical question... Assume a similar situation - i.e. a person is guilty is a lesser offence (e.g. failing to produce a ticket under the railway bylaws), but because of a lack of co-operation, ignoring letters, etc ends up being charged with a more serious offence (e.g. intention to avoid payment under the railways act). This person can show in court that they did not intentionally avoid the payment, so are not guilty of the offence they were charged with. Do they get off 'scot free'? Could the rail company still pursue them for costs via some other route? Could they still be charged with a bylaws offence - or is that 'double jeopardy'?
I disourage threads being taken off-topic, in compliance with this sub-forum's rules. If you wish to pursue this interesting question, then please start a dedicated thread.
It is common for Prosecutors to list more than one offence in the claim form sent to the Court (and which will then appear on the Summons to appear). Some Offences might not be pursued with evidence, while the most persuasive will be developed.
It is not uncommon for a Prosecutor to provide evidence in Court for an offence which is not as listed and Summonsed. Often the Defence will notice the 'switch', but sometimes they don't. I guess it is more often a blunder by the CPS than a ploy to secure a Prosecution on alternative grounds (e.g. the offence was commited every day of the week, the evidence is of the offence on Tuesday, and the Summons is for the offence on Thursday).
But to answer your strict question, then NO. It will never be possible for a Prosecutor who is struggling in pursuing a Prosecution for a RoRA Offence to switch, during the proceedings, to pursuing an alternative, Railway Byelaw, Offence.
Both the Court and the Defendant must be on proper notice of what the accusation is. I've often heard Judges proposing that the Offence being Prosecuted in front of them might be more successful if it had been Prosecuted as an alternative Offence, but that doesn't mean the Court has any jurisdiction to hear that alternative Offence without proper process. They won't.