I have to say that my ds was prone to throw himself into very violent rages at around this age. I never have got to the bottom of it, and for while blamed the fact that I spent alot of time in hospital and then a year in a wheelchair from him being about 3 and a quarter, so he didn't have the mummy he needed .
Rationally tho' I don't think that was all of it, he is very 'passionate' and I think his emotions overflowed well beyond his control. He got into bother at school for causing trouble, sometimes attempting to hurt other children. One day (he was 6) I had to go and get him out from under a desk for them (cos they are not allowed to handle the children )and bring him home, because he was in such a state of upset and anger. We never quite understood what set that one off.
I did stop all artificial sweetners, and aspartame now never touches his lips (nor ours). As he has grown up (now 8) he is calming, and such outbursts are very few and far between. He no longer throws things at us, but does direct his anger on himself, calling himself stupid, hitting himself on the head when we talk about the outburst afterwards.
I did learn that it is very important to be as calm as you can in the face of his anger. Don't shout back, it fuels the fire, and try not to physically restrain him if you can, as that frustrates him too, and makes you feel all the worse while you are fighting with him. You may get hurt too, which makes it harder to keep your own temper.
We would remove ds to his own room (quite a struggle), and I would shut him in when he was really in a strop. I had a lock put on the outside of the door. I know that sounds awful, but if I didn;'t lock him in he would follow me aroud the house, keeping up the tirade. Once shut in alone, he would start to calm down, and after a little while I could go into him, and give him what he needed, a strong cuddle (like swaddling a baby), and rocking until he was calm and cooler.
It was dreadful. School described him as emotionally immature, unable to deal with his feelings, which is difficult for us to handle as he is very bright, and intellectually advanced. From being 5 could hold quite adult conversations.
He's physically advanced too (a big boy for his age), and I believe that all his developmental energy went into his physical growth, and his intellectual growth, which left him no resources for his emotional growth.
I think he's catching up now, and we haven't had a major outburst for weeks.