You have not said whether this matter is being dealt with in the Magistrates� court or the Crown Court. Although the principles involved are similar, in practice slightly different things can happen. I am assuming, because the police are required, that your son is pleading not guilty and the matter has to go to trial. (If he had pleaded guilty, the police would not be required to attend).
Magistrates and judges are encouraged to take a robust line on adjournments. If your son does not attend for his trial it can be heard in his absence (and he will not then have the opportunity to put his version of events before the court). Similarly if prosecution witnesses are not in attendance the court is entitled to order that the matter proceeds. This may mean the prosecution has insufficient evidence to offer and the case will be dismissed.
If he has not done so he should engage a solicitor who will urge the magistrates or judge to order that the matter proceeds without further delay.