You're boyfriend is an idiot if he's seriously expecting to get way with a 'not guilty' plea on the grounds that "the guy was accidently hit 7 times over the head". If he pleads guilty as soon as he's given the opportunity to do so he could see his sentence cut by up to one third.
The actual document that the judge must refer to (when considering sentencing) is on pages 3 to 6 here:
http://sentencingcouncil.judiciary.gov.uk/docs/Assault_definitive_guideline_-_Crown_Court.pdf
It's possible, from your description of the incident and the victim's injuries, that it might only be regarded as a 'Category 3' offence, in which case he'll be looking at a sentence of around 4 years if he continues to plead 'not guilty'. (If it's 'Category 2' it would be around 6 years. I can't see it being classed as 'Category 1'). The actual time spent in prison, including while on remand, would usually be half of the nominal sentence but, for sentences of 4 years and above, it can be up to two thirds.
Anyway, if he really did consume 2 bottles of vodka he should consider himself lucky to be alive. Drinking just one bottle of vodka would kill most people. Only a very small percentage of people who drink two bottles survive.