There are a few things I don�t understand.
You have not said whether your son faced a trial, or if he pleaded guilty. You cannot do a �no plea� at the Crown Court. If you refuse to enter a plea there, a not guilty plea is entered on your behalf and you face a trial.
You can withhold your plea in the magistrates� court and the magistrates then decide whether to accept jurisdiction or whether to commit the matter to the Crown Court.
They obviously thought that the matter was serious enough for committal and the judge agreed (hence the sentence).
ABH is a serious offence (maximum 5 years, 7 years if racially aggravated) and not one that is charged readily without due consideration by the CPS.