No, he most certainly does not have to do anything of the sort.
Common Assault (Section 39) offences are among what are known as �summary� offences and as such can only be dealt with at the magistrates� court.
Nonetheless this is a serious matter and the offence carries a maximum of 6 months imprisonment. Your son should seek legal advice and review the evidence against him with his legal advisor.
If the events were as you described he will probably be advised to enter a not guilty plea. If he does so the case will be adjourned and a trial arranged.
The onus is on the prosecution to prove the offence and your son does not have to prove anything. However, if he does have any witnesses to support his version of events he may need to call them to give evidence.
Proper legal advice is imperative.