Well he can still go to prison whether he is under or over 18. He may go to a young offender institution to serve the sentence handed down to him.
I don't know whether it will be a youth court or a crown court, but really its the sentence handed down that matters. The fact that he is under 18 will be taken into account, but this rather has little impact, seeing that he has already been reprimanded AND given a final warning about his behaviour. This is proof that previous punishments have had little or no bearing upon his actions and so the courts will likely be quite harsh.
It is a great matter of importance as to whether he is being charged with GBH OR GBH with Intent, which is a far more serious crime. Also, you may wish to seek advice as to whether the injuried constitute GBH or ABH, as ABH is less serious.
For an adult GBH is punishable by any term not exceeding 5 years. GBH with intent is punishable by any term the judge seems fit. Please bear in mind that these sentences mentioned are only for the most serious of offences and it is unlikely that the sentence handed down will be anything like what can be given.
I would have though 4 months to 18 months youth offender institution.