Sorry to hear about your son being assaulted. You have every right to attempt to inform the magistrate of the new offence but the magistrate does not have to consider it. Due process is normally that the second offence is deemed to be unrelated as the first offence has gone past the conviction stage, so you must lodge a second complaint with the police. As the defendant has technically been convicted of assault and is awaiting trial, his second offence will be deemed to be more severe and will therefore warrant a harsher sentence that will be tagged onto the existing sentence.
PS I hope he gets custodial. Good luck.