I am sorry that my previous post was couched in such cryptic terms. The reason for this was that the £13.95 receipt tendered as evidence of the season's purchase on the day of the incident did indeed raise the possibility that your son had used a child ticket for an adult's journey. Whilst the appeal against the penalty fare barred prosecution for a 'travel with intent to avoid payment of the fare' offence under Section 5(3)(a) Regulation of Railways Act 1889 in relation to the homeward journey to Eastbourne, the statement lodged in support of the PF appeal plainly indicated that the morning journey outward from Eastbourne had been made with the same ticket. The Penalty Fares Regulations would have conferred no immunity from prosecution for that outward journey. Consequently, aside from the issue of the 50% discount flowing from the 16-17 Saver, the appeal documentation itself would have supplied all the evidence required to prove a Section 5(3)(a) offence in respect of that outward journey, and a prosecutor would have little difficulty identifying your son as the appellant submitting that evidence, as the uploaded copies do not conceal his identity. Given that material uploaded to this board is available for all to see, including railway prosecutors, I was reluctant to express my concerns in terms that might attract such a prosecutor's attention.
However, the fact that your son was entitled to apply a 50% discount to the cost of the weekly season by means of his 16-17 Saver puts a very different complexion on matters! The £13.95 receipt is consistent with purchase of an adult season with 50% 16-17 Saver discount, indicating that outward travel in the morning was made with a valid ticket and that the risk of prosecution for that journey does not arise. So we can return to the grounds on which a successful appeal against the PF might be mounted.
I agree with you that the erroneous statement on the PF Notice that it was issued to a child does not suffice as the basis for claiming that the Notice was not issued in accordance with the regulations, and that the amount of the penalty fare correctly includes the full single fare applicable to an adult's ticket. So, what other grounds are available?
Regulation 4(1) of the 2018 Regulations provides that:
“A person travelling by, present on, or leaving a train must, if required to do so by or on behalf of an operator, produce a valid travel ticket.”
I note that your son was approached for inspection of the ticket he had taken from the Eastbourne machine after the barrier had opened to permit his egress. I suggest that the opening of the barrier to allow egress must represent the end of the period in which a passenger can be said to be 'leaving a train', and that the obligation to present a valid travel ticket ended at that moment. If that is correct then there was no lawful demand for the ticket's production and no right to charge a penalty fare when a ticket valid for the completed journey was not produced. In that sense the penalty fare was not charged in accordance with the Regulations' requirements, and this will be a valid ground for appeal under Regulation 16(3)(a).
I also consider Regulation 16(3)(c) to be an applicable ground for appeal:
“the appellant owns a season ticket valid for the journey in question but was not in possession of the season ticket at the time the penalty fare was charged”
That seems to me to capture exactly the circumstances in which your son found himself: he had that morning purchased a season ticket valid for the journey just completed (the documentary evidence supporting this lies in the receipt in conjunction with the 16-17 Saver railcard), but was not in possession of it due to the Eastbourne barrier, for whatever reason, having failed to disgorge it.
I also note that Regulation 16(3)(c) specifically contemplates 'ownership' of a season ticket, which I take to involve recognition that a season ticket-holder has the right to own and keep such ticket. Where a railway operator, confronted with a passenger's plausible explanation that its equipment has retained a ticket he owns, declines to take reasonable steps to investigate the position and restore the ticket to its owner if found within the machine, it must bear the consequences of that refusal. In principle the railway's refusal to attempt restoration of the ticket to its owner might formerly have been actionable in detinue, but this is a tort (legal wrong) that was abolished by the Torts (Interference with Goods) Act 1977 and would now appear to be actionable instead as the tort of wrongful interference with goods belonging to the ticket-holder, for which he may recover damages representing the extent of the loss flowing from the ticket's wrongful retention by the railway (for a 10-journey season used to make 2 journeys before its loss to the ticket-holder, four-fifths of its purchase price).
I suggest that a claim for partial refund of the weekly season's cost should initially be made to the company managing Eastbourne station. Regretfully, I don't have much confidence that the appeals body will correctly uphold either a second stage or final appeal against this penalty fare, but in the somewhat unlikely event of the operator subsequently pursuing the penalty fare through the civil courts if it remains unpaid, wrongful interference with goods is potentially available as the basis for a counterclaim and set-off.
Automerged addition:
On re-reading the thread I note from post #11 that the exit barrier did not open after the season was inserted and that this is apparently the reason why railway staff approached. That considerably diminishes the force of the argument there was no lawful demand for production of a ticket, to the point that it is not one that I would pursue. The Regulation 16(3)(c) ground of appeal still appears to me to hold up. The appeal panel should be invited to draw the appropriate inference from the fact that the official who issued the penalty fare declined to take any steps to verify the explanation offered by the passenger when invited to do so, notwithstanding the availability (and production?) of a ticket purchase receipt issued on the same day that was consistent with the explanation given. Are any previous weekly season tickets available as evidence that it is the OP's son's practice to make regular use of these?