Thanks Nosha - but there are far more experienced people on here than me who deal with criminal law. The reason this has to go to the Crown is the type of offence. GBH is indictable only, which means it can only be tried in the Crown Court.
An early guilty plea will result in up to a third off his likely sentence. The only reason this has been mentioned is that it is standard. He should not plead guilty to something he did not do.
The phone call is unfortunate, but never going to be decisive of guilt. It is you talking to his father who is a family friend. At best it is hearsay and will be given such weight. Given that you did not admit the allegation, nor deny it, I fail to see the relevance in any way. You also have an explanation as to why it was denied. It can be used in court.
You can be a witness to the defence - you say you were not there so are you able to provide a supporting witness to say where you were? Your wife can also be a witness, as can the girlfriend. I am intrigued about the medical evidence - given that the alleged victim says it was caused by a punch, yet the medical evidence says not likely. Who has given the expert medical evidence? Your son does not have to prove how the injury was caused - the prosecution have to prove beyond reasonable doubt that your son caused the injury in the way they say.
As for getting advice, at the PCMH (Plea and Case Management Hearing) which will be the first hearing at the Crown Court, your son will be represented by a barrister. It is at this stage you should be able to obtain some advice. Ideally you should have a conference with counsel beforehand and you should request this from your solicitor.