Thank you everyone for your replies they are really helpful as I am in full panic mode. I have attached the letter sent by train company and my son assures me this is the first time this has happened. I am more than happy to pay any amount for fine but just to do this the correct way in order to avoid going to court.
First time he has been caught or the first and only time he has ever done this? People on here who fare evade seem to find it difficult to make that distinction. I'm not syaing your son is a liar but may not see the difference...
This is REALLY important - if he has done something like this before chances are they will find out - esp if tickets bought on line. He has short fared and bought child tickets when an adult - it's statistically unlikely he got caught on the first time he did either or both of these things. he needs to be 100% honest about this - as if they detect any lie the prospects of prosecution are increased
But if they are not asking about other occasions now don't mention them at this stage, but don't say 'never done this before' either unless that is 100% guaranteed.
Anyway - see the post from Hadders at post 5 and get your son to start drafting up something that is along those lines - see other threads for plenty of examples of other people efforts. Get that drafted and post it up here for people to help get it as good as it can be. If he is at University I am sure he can write what is needed.
There won't be a fine as such - what your son needs to do is basically ask them if they will be kind and sympathetic enough to allow him to settle the matter out of court for a sum that represents the fares evaded and the costs of their admin. All the cards are in their hands, but they do often settle in this way. This then avoids the criminal record that will result for court action
Also - get him to buy a Railcard NOW. This is then something that he can mention in the reply to show that he has taken steps to legitimately reduce his fares in future without breaking the law, helping support the point that he has learned his lesson and will not evade again.
Finally do not miss the deadline for response, send your reply by tracked Royal Mail postage too - you need to guarantee they get it.