A single blow without an implement is extremely unlikely to qualify as 'Section 18' (= "with intent") unless, for example, your b/f deliverately targeted his victim (e.g. by going round to his house to attack him). So that means that it's probably the lower 'Section 20' charge.
A broken jaw is probably at the lower end of 'harm' within the range of GBH offences. If it was just a pub argument (or similar), with no premeditation and no racial overtones (or an attack based upon the victim's sexuality), 'cuplability' is also 'low'.
That means that it's a 'Category 3' offence. If the judge agrees with that, he must use a 'starting point' sentence of a 'high level' Community Order and then move the sentence up or down depending upon all of the circumstances made known to him. (He can go down to a 'low level' Community Order or up to 51 weeks imprisonment). So I agree with others that a custodial sentence might well be avoided.
However if the judge disagrees with my assessment of both 'harm' and 'culpability' as being 'low', and places one of them as 'high', then it's a Category 2 offence, with a 'starting point' sentence of 18 months imprisonment and an available range of between 1 year and 3 years imprisonment.
See pages 7 to 10 here for full information:
http://sentencingcouncil.judiciary.gov.uk/docs/Assault_definitive_guideline_-_Crown_Court.pdf