If no weapon was used and the assault was not pre-meditated, the 'starting point' sentence is 24 weeks custody. The judge must start from that point, consider all of the circumstances and then (unless there are genuinely exceptional circumstances, which must be explained in open court) move the sentence to a point not exceeding 36 weeks custody and not below a 'high' level community order.
However those sentences refer to a first-time offender convicted after a trial. If your son has previous convictions, the possible sentencing range could be higher. If he enters an early guilty plea, any sentence can be cut by up to one third.
So it's clear that a period of imprisonment is the 'normal' sentence for a Section 20 conviction but, if your son has no previous convictions (or only convictions for minor, non-violent offences), didn't use a weapon and can convince the court that he was genuinely provoked, he stands a reasonable chance of avoiding custody.
Source:
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
(Refer to the table on page 15)
Chris