As with all assault offences, 'common assault' is placed into one of three categories by the courts (with Category 1 being the highest level and Category 3 being the lowest). A custodial sentence CAN'T be handed down by a court for a Category 3 or Category 2 offence. Even for a Category 1 offence the 'starting point' sentence (from which magistrates have to work up or down) is still NON-custodial.
So it's only offences at the VERY top end of Category 1 which could see someone who's been convicted of common assault sent to prison. That would require 'harm' to be at the highest point consistent with a charge of 'common assault' (rather than 'ABH') and 'culpability' also to be at the very top end of the scale.
e.g. someone who deliberately sought out a vulnerable person to 'rough them up a bit', resulting in the victim being afraid to leave their home, might end up in prison for 'common assault' because 'culpability' was extremely high (i.e. it was a planned attack on a vulnerable victim) and 'harm' would be classed as 'high' too (because of the psychological effect of their actions).
So, other than the really extreme type of circumstances in my example above, it's simply not possible for a court to send someone to prison for common assault. Your son will almost certainly end up with nothing more than a community order if he's convicted.
See pages 23 to 26 here to see exactly what the magistrates will have in front of them:
https://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf