GBJOCE: one thing to note in relation to a section 20 GBH is the requirement of intention or recklessness. If your son can prove that he was so intoxicated that he was unable to formulate an intent to assault the victim then the court could consider recklessness. The case of R v Lipman outlines the defence of Intoxication, though i would look into it further before saying if it would apply to your sons situation or not.
The fact that his actions were not aggressive shows that he might be able to prove he had no intention of committing this crime. But the fact that his actions were stupid could quite well mean that he was reckless. The test the courts would use is objective in that would an ordinary person realise that his actions could cause injury.
Finally your son not having any previous convictions might call for an application of good character. Any good lawyer would use what is called the "Vye direction". But be aware that the minute defence counsel acting for your son assert good character, the prosecution will have an opportunity to cross examine your son about any bad character.. ie. if he has a previous caution (which is not a conviction) then the courts will indeed hear about this.
On a final note..something i tell a number of clients.. the prisons are nearly full. The courts are under an extreme pressure to look at alternatives to imprisonment, eg. lower sentence plus licence periods, or hefty fines etc.
The magistrates court appearance is merely a pre-trial review, which means in effect no evidence is heard but because of the seriousness of the offence an application is made to hear the case at the crown court. There is also an opportunity for the Crown to downgrade your sons charge, ie. though the victims injuries are serious, from my personal experience they are on the borderline of G B H. Thus if your son is lucky, the crown might well change his charge to a section 47 A B H, which is not as serious as the