ABH offences are divided (for the purposes of sentencing) into 3 categories. Judges must consider both 'harm' and 'culpability' and decide whether each is 'high' or 'low'.
'Harm' will be 'high' if the injuries, or psychological effects, resulting from the assault are relatively serious when compared to ABH offences as a whole. 'Culpability' will be 'high' if the offence is related to the victim's sexual orientation or to his disability or if a weapon was used in the assault, which includes kicking the victim or headbutting him. (Other factors may also apply in determining whether 'culpability' is 'high' or 'low').
If both factors are 'high', it's a Category 1 offence, with the judge having to work up or down from a 'starting point' sentence of 18 months imprisonment and not being able to go below 1 year or above 3 years.
If only one factor is 'high', it's Category 2, with a 'starting point' of 6 months imprisonment and a range going from a 'low level' Community Order up to 51 weeks imprisonment.
If both factors are 'low', it's Category 3, where a custodial sentence isn't available to the judge. The 'starting point' is a 'medium level' Community Order, with the range going from a fine up to a 'high level' Community Order.
So it should be clear that the likely sentence your boyfriend will receive for ABH will depend to a very large extent upon which category the judge sees the offence as falling into. (Given that he's got previous convictions for violence, and that he pleaded 'not guilty', it's very likely that the sentence will be near to the top of the appropriate range. It's also unlikely that a suspended sentence would be passed).
So, solely with regard to the ABH offence, if it's 'Category 3', he's safe from a custodial sentence. If it's 'Category 2' then he's almost certainly going to prison, perhaps with a 10 month sentence.
Affray is far trickier to advise upon as there are no official sentencing guidelines. However judges will look at case law to see what sentence might be appropriate. The following is a cut-&-paste from the CPS website:
"R v Fox and Hicks [2006] 1 Cr. App. R. (S) 17 CA
Football match. Opposing fans, threats, debris thrown. Guilty pleas, albeit on first day of trial.
H, age 25 had relevant and very recent pre cons and was on bail at the time of the offence. He pleaded provocation.
F, aged 35 and a family man, was of good character and had participated to a lesser extent.
F was sentenced to 8 months and H to 12 months."
As that example illustrates, a conviction for affray is very likely to result in a custodial sentence and, even if the ABH is only 'Category 3', that's where your boyfriend's biggest problem might lie.
My guess (and it can be no more than that because I've not got the full facts to hand) is that the affray conviction might result in imprisonment for perhaps 10 months. If the ABH offence is 'Category 2' then that might also result in the same sentence, probably to be served concurrently. (If the offence is 'Category 3' then it's largely irrelevant here. If it's 'Category 1' then a sentence of 2½ years, to be served concurrently with the affray sentence, might be passed).
So (given that offenders are normally only 'banged up' for half of their nominal sentence) my guess is that your boyfriend will actually spend around 5 months in prison.