Ghosty, ds1 was like this, and to an extent still is like this, in that he does seem to feel emotions more strongly than other children, and struggles to control them: anger, fear, worry, excitement, happiness, outrage at 'injustice' ...
But he did turn a corner when he was 5.5. The first time he "lost it" at school they came down on him like a ton of bricks: he had long serious chats with the Head (who has the patience of a saint, she is wonderful) and the Deputy Head, etc. etc. And they just made it clear to him that this was not acceptable behaviour in their school, that he was 5 now, and he was capable of keeping a lid on it. They put in place loads of positive praise/reward stuff ... and it worked! He was clearly at a point developmentally that he was actually capable of keeping control, and someone (not me) was able to get across to him that he had to. I do believe that this was and is more difficult for him than for most children, and he still gets ridiculously excited about things (he's almost 8 btw), but his self-control is much, much better.
Teaching him to count to 10 in his head did help a bit I think. But other 'substitutes to express his anger' like drawing an angry picture, or singing an angry song, or stamping his feet, didn't actually work for him, but apparently they do for some children.
Another which can help is to write a little cartoon story of a child losing their temper, with speech bubbles and so on, and - most importantly - 'think bubbles' of children/adults who are observing the incident. You can look at this with him, and maybe leave the think bubbles blank and ask him what the people would be thinking. (I'm not sure whether a 5 yr old could do this though.) But either way it might help him to see the situation through someone else's eyes, which after all is the key to learning to behave in a socially acceptable manner, rather than just doing what suits you.
Sorry it's a bit rambling - I'm just throwing out thoughts here. Hope something helps you and ds.
Roisin