The court needs to consider both 'harm' and 'culpability'. With the situation you've described it seems likely that both of those factors will be seen as falling into the 'lower' range. If so that makes it a 'Category 3' offence.
The 'starting point' sentence (from which the court has to work up or down) for a Category 3 offence under Section 20 is a high level community order. Although a custodial sentence is available to the court for a Category 3 offence, an early guilty plea and a previously clean record should ensure that only a community order (involving unpaid work) will be imposed.
If, for some reason, the offence is viewed as 'Category 2' then the court is obliged to impose a custodial sentence. However such a sentence can be suspended if it doesn't exceed 2 years (which it wouldn't with an early guilty plea and no previous convictions) and that would probably be the most likely outcome in such a case.
As I stated to start with though, I can see nothing in what you've written to indicate the offence would be regarded as anything other than Category 3, effectively ruling out a custodial sentence anyway. (I only mentioned sentencing for Category 2 in case the court is presented with some evidence which you've not provided us with).
So your boyfriend should prepare himself for many long hours of unpaid work.
https://www.sentencingcouncil.org.uk/offences/crown-court/item/inflicting-grievous-bodily-harm-unlawful-wounding-racially-or-religiously-aggravated-gbhunlawful-wounding/