The Judge has to consider both 'harm' and 'culpability', to decide whether they should each be classified a 'high' or 'low'. If both are 'high', it's a 'Category 1' offence. If only one is 'high', and the other is 'low', 'it's Category 2'. If both are 'low' it's 'Category 3'.
The use of a weapon and/or kicking someone in the head would normally ensure that 'culpability' is to be regarded as 'high' but a mental disorder might see it treated as 'low'. A sustained assault could see 'harm' regarded as 'high' (as could an injury which was just below the level necessary for a 'GBH' charge).
It's likely that the Judge will regard the offence as (at least) 'Category 2', meaning that he has to work up or down from a 'starting point' sentence of 6 months. A non-custodial sentence would still be possible but a maximum of 51 weeks is also possible. However those sentences apply to a first-time offender, convicted after a trial. If your ex was convicted in 2010 that could push sentencing higher but his fairly early guilty plea this time could see it pushed lower.
The actual document that the Judge must consult is here:
http://sentencingcouncil.judiciary.gov.uk/docs/Assault_definitive_guideline_-_Crown_Court.pdf
(See p11, et seq).
The Judge will have access to far more information than I have but I'd guess at a sentence of around 6 months. If so, he'd only have to spend 3 months in prison, meaning that he could be released a few weeks after sentencing.