'Assault by beating' is simply an alternative charge to 'common assault' and, as such, is the very lowest level of assault charge.
While it can theoretically lead to a prison sentence that can't happen unless the magistrates view the offence as at the very top end of 'Category 1'. That would require both 'harm' and 'culpability' to to be seen as 'high. (e.g. where the offender had deliberately sought out a vulnerable person to attack them, leaving them greatly distressed for a long period of time). The type of offence you've referred to will fall into either Category 2 or Category 3, meaning that your son CAN'T be sent to prison (which is probably what's worrying both him and you).
The most likely outcome will be a community order (i.e. some unpaid work or similar) or, if it's only seen as a Category 3 offence, possibly just a fine.
See pages 189 to 192 here to read exactly what the magistrates will have in front of them when considering their sentence:
https://www.sentencingcouncil.org.uk/wp-content/uploads/MCSG-April-2017-FINAL-2.pdf