Judges are obliged to pass sentences within fixed guidelines unless there are genuinely exceptional circumstances (which the judge must explain in open court).
The 'starting point' when sentencing a first-time offender who has been found guilty (after a trial) of a Section 18 offence where the victim "suffered life-threatening injury or particularly grave injury (where the offence was not pre-meditated)" is 8 years imprisonment. Unless genuinely exceptional circumstances apply, the sentence can't be below 7 years or above 10 years.
However an early guilty plea can see the sentence cut by up to one third. So if the judge doesn't see any 'genuinely exceptional circumstances', the lowest sentence that could be passed would be 7 years minus one third = 4 years 8 months. Offenders are usually released half way through their sentence (although, for sentences of over 4 years, that's not always the case), so your son would actually spend 2 years 4 months in prison.
My feeling is though that the judge might well see the circumstances as falling within the 'genuinely exceptional' category, and use the lower sentencing band, which applies where the victim has "suffered a very serious
injury or permanent disfigurement". The 'starting point' is then 5 years, with a 'range' of 4 to 6 years. Taking the lowest possible point of 4 years, and knocking a third off for an early guilty plea, gives a sentence of 2 years 8 months (of which 1 year 4 months would actually be spent 'inside').
Source:
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
(See the table on page 13)
Chris