Here's the actual document which judges are obliged to refer to when passing sentence. (See pages 3 to 6):
http://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf
(A judge can only go outside of those guidelines when there are very exceptional circumstances, which he must explain to the public in open court)
While the use of a weapon might normally push 'culpability' into the 'higher' category, 'excessive self defence', 'provocation' and 'lack of premeditation' could well be enough to keep it in the 'lower' category.
It's hard to say whether 'harm' would be seen as fitting into the 'lower' or 'higher' category. Any GBH offence must involve some fairly serious injuries (otherwise it would only be ABH), so even a fractured skull and jaw doesn't automatically mean that the judge would see the injuries as at a 'higher' level of harm. However if they led to an extended stay in hospital, or life-changing effects (such as a permanently disfigured jaw) then he probably would.
So, assuming 'culpability' is seen as being at the low end of the scale then it's either a Category 3 offence (if 'harm' is also seen at the same level) or Category 2 (if 'harm' is viewed by the judge as 'higher'). If it's Category 3 the sentence is likely to be around 4 years imprisonment. If it's Category 2 it could be 6 years.
(Note 1: If your son pleaded guilty at the earliest opportunity there would be a one third discount from those sentences.
Note 2: The actual time an offender is 'banged up' for is half of the nominal sentence)
So, assuming that he pleads not guilty (on the grounds of 'self defence', although I can't see that standing up in court anyway), your son could be 'out of circulation' for around two to three years.
It's possible however that his solicitor/barrister will be trying to get the 'Section 18' charge reduced to 'Section 20' (i.e. getting 'with intent' thrown out, based upon the grounds of self defence). That might stand a far better chance than a simple 'not guilty' plea. If your son was to plead 'not guilty to Section 18 but guilty to Section 20', and the court accepted that plea, he would be sentenced on the basis of (probable) Category 2 Section 20 offence. (See page 12 of the document I've linked to above). The starting point sentence for such an offence is only 6 months custody and his 'guilty to Section 20' plea would see that reduced to 4 months (with only two months actually to be served in prison). It's also possible that a suspended, or non-custodial, sentence could be passed.