Thanks everyone for all your advice, I'm very grateful - and some good ideas here. It's very rare for my son to undertake unusual journeys on his own, and couple that with living through a pandemic which is making life unpleasant and difficult, we just stumbled upon a set of circumstances which will probably never happen again. I've have surprisingly, just received a reply from them to my mail I sent through their address for submitting evidence - no message, no answers to my questions, just another copy of the same letter asking for evidence! Annoying, but at least a bit of ammunition for me to use - I tried to explain our circumstances and the difficulties we would have getting written evidence and they have effectively ignored me and sent me back to where I started. If this ever gets resolved, I will let you know.
Thanks for update - sadly not surprising. Inflexible adherence to policy at this stage - this is way you will have the frustration of having to keep at it and be prepared to go for an Ombudsman type review at the end of the day - they just don't seem to read and act on stuff unless you send exactly what they want as it were.
Juts reply as required, keep copies of everything you send / screen shots if needs be etc.
EG at this stage write to them saying
'I will seek copies of the proof of disability you require form an appropriate source / agency - ie GP / DWP, I am sure you will understand that this may take a period of time for me to secure. I would be grateful if you could acknowledge this at this stage'
and plough on (this is the sort of thing that you need to to counter any allegation that you ignored their requests to you, if you have to send the papers to the Ombudsman at a later stage for example).
Tho I would suggest Transport Focus would be a better body than the Rail Ombudsman based on experience detailed in other threads here
Transport User Advice from Transport Focus.
www.transportfocus.org.uk
Are you up against an Appeal deadline date (presume yes, if so what is it?*) and maybe can others advise that if there is a delay whilst sourcing the proofs is the clock still ticking to that deadline or is it deemed to be extended.
This of course being the alternative to the course of action in Post #15 (pay up now, argue about it subsequently).
* Apols I see you have just 7 days to submit the proofs
If you want to pursue the Appeal I'd be writing to them as above now (sent registered post) making it clear that you have to source the proof of disability and may be unable to secure this by the deadline given you are reliant on relevant agencies (NHS / DWP) to provide the documentation required. You trust that they will keep the case open and not take further actions, and can they reply to confirm this.
PS - also I'm not familiar with London Freedom passes, but assume this awarded to your son as a result of disability - if it is clear of that on the card is it possible they would accept an image of this as proof of disability? Is that worth a try - my fear is they would think you are trying to show it as proof of having a ticket - which you would not be, leading to another frustrating exchange....ie they would look at the image not any message - but you could try writing on a piece of paper
'this image of my London Freedom Pass awarded in respect of my disability is submitted as proof of my disabled status and not submitted for the purposes of consideration as a ticket for the journey' put the card next to that message, photo it and upload that?