The Crown Prosecution Service website defines ABH (which might well be the relevant charge) here:
http://www.cps.gov.uk/legal/section5/chapter_c .html#09
Scroll down to the next two sections, on that page, to view the definitions of GBH, and GBH with intent.
If your boyfriend's account of events is accepted by the police (or CPS, or court, depending upon how far things get before the offence is classed as ABH), he'll probably get a non-custodial sentence such as 100 hours unpaid work. That, of course, assumes that he hasn't got a string of previous convictions for violence. (If so, or if the court decided that the matter was rather more serious than described, but still ABH, a sentence of 6 months imprisonment might be imposed).
If the police and CPS believe that there is sufficient evidence to secure a conviction for GBH with intent, and a jury convicts your boyfriend, a custodial sentence would seem to be inevitable. (The maximum sentence for GBH with intent is life imprisonment, although your boyfriend would probably receive a sentence of between 2 and 4 years).
Chris