Assuming that he didn't use any form of a weapon (and that the assault was not premeditated), the 'starting point' sentence is a 'high' level community order, with a sentencing range running from a 'medium' level community order up to 6 months imprisonment.
However those sentences apply to a first-time offender, convicted after a trial. An early guilty plea can see a reduction in sentence but previous convictions can push the sentencing range higher. (The maximum penalty for ABH, other than when it's racially aggravated, is 5 years imprisonment).
With previous convictions for assault your ex is probably facing a sentence of around 6 months imprisonment if he's found guilty after a trial, or 4 months if he pleads guilty at the earliest opportunity.
Chris