Dom R is making a very good point. It is his behaviour that you have told his parents about. If he doesn't want them to know about that then he should stop doing it. also if he doesn't want them to know, doesn't that prove to him that his behaviour is wrong?
I think you've made the job much easier for him to turn to his parents now. his parents can now be of real help now they know the whole the story (do they know everything now? I'm guessing there is still stuff he should tell them). Especially as it sounds like he doesn't want his parents to know about the aggressive behaviour, therefore implies that he had no intention of telling them. how can they be of help when they only know half the story.
Remember you did nothing wrong as this sort of thing can turn into full on violence and it's important that his parents are told in case it happens again. plus it sounds like you have a good relationship with his parents and they would be upset if they didn't know about such a huge issue with their own son because you didn't tell them (why hadn't h already spoken to them anyway?- sounds like he can do with their support- I'm wondering if he has any intention of turning to them for support. if he hasn't already done so, he may be just saying that "he can't talk to them when he's ready to" but in reality has no real intention of telling them any more than 20% of the story).
