Let's deal with this bit first:
"
nephew thought he was going to get hit so hit him first"
'Self defence' is only a valid defence in a court of law if the absolute minimum amount of force was used, when it was impossible to simply get away from the other person.
If your nephew could have simply run away, 'self defence' won't carry any weight in court. If you nephew could simply have pushed the other guy away, giving your nephew time to run away, 'self defence' won't work. If your nephew could have used less force, or used a blow which was less likely to cause real injury (such a punch to the midriff) 'self defence' is meaningless.
Your nephew should contact a solicitor and ensure that he attends the police station with him. GBH (when charged under Section 20 of the relevant legislation, which would seem to apply here) is an 'either way' offence. That means that it can be dealt with in a magistrates' court or the magistrates can refer it up to the Crown court. (That usually occurs when the magistrates believe that their sentencing powers might be insufficient). In practice, the majority of GBH cases end up before the Crown court. Unless your nephew's legal team are 100% certain that he acted within the
legal definition of 'self defence' (and not just because he thought of his actions as defending himself), he should plead guilty.
Please see my post, here, regarding the likely sentence:
http://www.theanswerbank.co.uk/How-it-Works/Qu estion639685.html
Chris