There's an inconsistency in your post. A 'minor' injury can't lead to a Section 18 charge. Read the definition of 'Section 20 GBH' here (in particular paragraph 55) to see the type of injury that the Crown Prosecution Service say your son has inflicted:
http://www.cps.gov.uk...on/#Unlawful_wounding
Then scroll down to the next section to see why the charge has been increased to 'Section 18'.
The 'starting point' sentence for a @section 18' offence involving the use of a weapon (where the assault was not premeditated and the victim did not suffer life-threatening or very grave injuries) is 5 years imprisonment, with judges expected to pass a sentence between 4 years and 6 years. However those sentences refer to a first-time offender, convicted after a trial. Previous convictions can push the sentencing higher but an early guilty plea can see the sentence cut by a third.
At a rough guess the judge will opt for a 6 year sentence but (assuming that your son pleads guilty) then cut it down to 4 years. Offenders normally spend half of their sentence in prison, so it's likely that your son will actually be 'banged up' for about 2 years.
Chris