Here's the actual document that the judge must refer to:
http://sentencingcouncil.judiciary.gov.uk/docs/Assault_definitive_guideline_-_Crown_Court.pdf
My guess is that the assault will be classed as a 'Category 2' offence which means, as Black_cat51 suggests, a 'starting point' sentence of 6 years imprisonment. (The sentences shown on page 5 are those for fist-time offenders who are found guilty after a trial, so they would seem to apply to your brother).
Someone who is sentenced to 6 years imprisonment can be considered for release from prison after 3 years. If he is not released at that point he'll be released after 4 years.