(2-part post):
There seems to be some inconsistency between the injury you state that the victim sustained and the charge that you say your boyfriend is facing.
'A couple of stitches' can't lead to a conviction for GBH. Read section 5(v), on the Crown Prosecution Service's website, for examples of the type of injuries that constitute GBH:
http://www.cps.gov.uk/legal/section5/chapter_c .html#10
If the victim really did only require a couple of stitches, your boyfriend's barrister should be seeking to get the charge dropped to ABH. See section 4(ii) for the type of injuries which are classified as ABH:
http://www.cps.gov.uk/legal/section5/chapter_c .html#09
If the charge was dropped to ABH, your boyfriend might well receive a non-custodial sentence (although it's far from certain). However, it's much more likely that the CPS have correctly categorized the victims injuries. If so, it's important to consider whether your boyfriend has been charged under Section 20 or Section 18 of the legislation. Section 20 offences carry a maximum penalty of 5 years imprisonment, whereas Section 18 ('GBH with intent') offences can result in life imprisonment. The use of a weapon can often lead to a Section 18 charge. See 6(vii) here:
http://www.cps.gov.uk/legal/section5/chapter_c .html#11