Hello. I am a solicitor with 20 years experience. I will try to answer this for you.
ABH stand fot actual bodily harm which means some harm more than refiling but not so serious as grevious bodily harm. Assualst causing bruises, black eyes that sort of thing. The maximum sentence for ABH that is not racially aggravated is 5 years so he would seem to have received the maximum possible sentence for this offence. I have not in over 20 years of practice com across a client receiving the maximum sentence for ABH. Also prisoners will usually serve half of their sentence in jail and not 60% as he appears to have done.
Appeals normally have to be lodged within I within 28 days of sentence and ordinarily would have concluded by now but it may have been referred to the Criminal Cases Review Body to decide if there may have been a miscarriage of justice.
I think you have reason to be sceptical about what you have been told (I am not saying he is lying) and you should ask the probation office anything you want.
If you follow this link it will take you to the government site re emplyers geeting CRB checks on employees.