If it is a first offence the court will be lenient, your son on the advice of his lawyer should tell the guards he accepts full responsibility, so he may just get a caution. If the assault was severe then that may not be possible of course.
But the guards and court will want to see remorse, understanding of what they did was wrong and mitigation for the damage, either a monetary offer, anger management, counselling, stopped drinking, etc. whatever is relevant.
With a file going to the dpp it sounds as if your son has not admitted to anything so I think he should start there. Not putting the guards, victim and court system through the time and expense will help with mitigation. You can advise him but he will ultimately need to decide himself, risk court or take steps now. If you think his actions were typical of his character then maybe he needs to see the consequences before he understands his actions. But maybe he should have acted already before the guards sent the file off?
Approaching the victim is not a good idea at all, it could be seen as interfering with a witness and shows poor understanding of the legal system. If he wants to apologise he will have to do it through the guards.
As a parent you probably need to carefully consider your response to this with your son. It can be too easy to be persuaded he was provoked, he is never like this, life was against him etc. What do you want him to learn from this? Can you help him to honestly assess his actions and look at what type of person he wants to be in the future.
