I responded to your post on the Relationships board last night - see here: www.mumsnet.com/Talk/relationships/1529767-help
Victim Support operate a dedicated service for young people who are in your position. Please make contact with them and let them help you through the possibiity that you may be required to give evidence in Court if he maintains his not guilty plea.
But please note that the fact that he has maintained a plea of not guilty to date doesn't mean that he will plead not guilty on the day of the hearing in the Youth Court.
By pleading not guilty on the various Court appearances he will have made for his bail to be extended until the date for hearing was set, he's been able to remain at liberty.
However, he will be advised that if he pleads not guilty on the day and is found guilty by the Court, it's probable that he will receive a more severe sentence than if he pleads guilty.
In addition, if he has previous conviction his offending history will be taken into account if he's found guilty of offences against you and this, together with other factors, will determine his sentence. If he's offended before, it's probable he will receive a custodial sentence.
Nevertheless, you need to be prepared for whatever may happen on the day and Victim Support's Witness Service is best placed to support and advise you and also your mum who, presumably, will be accompanying you to Court.
I know you've said you find it difficult to talk to your mum but she may be finding it equally difficult to talk to you, especially if she thinks that you are still entertaining fond feelings for this violent and abusive young man.
It's not easy being in the grip of powerful emotions in respect of romantic relationships at any age, but it's especially difficult during early teenage years when it's not unusual to believe that you're an adult who is fully capable of determining your own life but you're unfairly being treated like a child by parents/teachers etc.
Please keep posting on your thread on Relationships (see above link) - the majority of the responders on this site are mums but you won't be judged for having done what so many thousands of women/young girls have done before which was to get in over your head with a male who failed to show you the respect you deserve.