If the only injuries sustained were cuts simply requiring stitches (rather than needing a transfusion), a charge of 'ABH' would seem to be the most likely outcome. See paragraph 46 here for illustrations of what counts as 'ABH':
http://www.cps.gov.uk...he_person/#P189_14382
Then scroll down to paragraph 55 to view the same thing for 'GBH'.
Sentencing guidelines for 'Section 20' are on page 15 of this document:
http://www.sentencing...person_accessible.pdf
Scroll down to page 17 for 'ABH'.
The sentences shown in that document refer to a first-time offender who is convicted after a trial. An early guilty plea can see a reduction in the sentence (of a third, in the case of a custodial sentence).
If your son is convicted of Section 20 (while using a weapon, i.e. the wine bottles), he'll probably receive a 12 month prison sentence, cut by one third to 8 months. Only 4 months would actually be spent 'inside'.
If the charge is dropped to 'ABH' he'll almost certainly receive a non-custodial sentence.
Chris