Son received Fixed Penalty Notice for £90 plus £9.60. Son travelling with girlfriend, she has a railcard and bought them both a ticket. She did not realise until the conductor came round that the railcard discount had been applied to my sons ticket too. He offered on the spot to pay the difference or buy another ticket, the conductor refused. The railcard discount was max £5. Today he has received a Fixed Penalty Notice for nearly £100. He is an 18 year old school student, it was a genuine mistake which he offered to rectify on the spot. Any advice on next steps, appears you can not appeal this.
As others have said, it sounds like this is an out of court settlement offer (some train companies may use fancy terms for this).
If you choose not to pay it, the company may prosecute; if you choose this course of action I would ensure a solicitor is appointed to represent your son. If the prosecution is under the draconian Railway Byelaws, this is a strict liability matter, so it maybe difficult to defend the case. That said, some train companies have been known to drop cases when they realise the matter is going to be defended by a solicitor, as most of the cases that are brought before the courts do not appear to be defended and the train companies can easily rake in loads of money using very little court time in such instances.
It may be better to pay the amount demanded, to make the issue go away.
In theory you could try to negotiate the amount down, but the amount of effort it would require to get through to someone, combined with what is probably a relatively low chance of success, probably means it's not worth doing that.
The train company concerned does not care about customer service and does not want your custom or your son's custom and won't be interested in acting in a reasonable manner, so the above are your only realistic options.
Sorry this isn't what you wanted to hear but that is the harsh reality I'm afraid.
If the website in question does not make the Railcard situation clear, you could contact them about the matter, and you could try to seek some recompense but I wouldn't hold out much hope of that.