I found the book The Male Brain helpful for understanding my sons' behavior.
One thing I remember in particular is to expect a behavioral shift around 9 years, as a boy starts asserting his own authority (by making decisions and accepting consequences, a marker of self-confidence). He stops identifying as a little kid, wants to join adult or older child groups, and rejects associations with smaller kids in an attempt to demonstrate that he is ready to be an adult.
Because of his inexperience, his choices are sometimes wrong, but now he knows he is responsible for his decisions. At this stage in his life, he must learn to deal with disappointment, anger, and embarrassment as a result of his own choices ("All the kids are laughing at me for wearing this stupid shirt") where in the past, he may have been able to pass that responsibility off on someone else ("Mom made me wear this stupid shirt, so it's her fault the kids at school laughed at me").
Because asserting his will means asserting his independence, this time of life often means conflicts between father and son, as a boy simultaneously models his father's behavior and rejects his father's authority in favor of greater autonomy. In other words, the boy wants to act like a man, and can feel like his parents' rules treat him like a baby who can't make his own decisions.
It sounds like you have done a great job raising your son, and the most likely trigger for his rages are embarrassment and feeling childish. It may help to explicitly tell him that you know what it is like to feel enraged sometimes, and explain what you do to help tame that behavior. If he hears from you that even grownups must deal with feelings like his, and that it is not an easy thing to do, it may help him feel less like a "baby" for getting upset. At the same time, it will give him tools for communicating his feelings and coping with them.
Lastly, spending some one-on-one time with him may help, particularly if you can help him feel that you respect him and are proud of the man that he is growing into. It may be that he is upset because he is treated like a kid (even though he is one) since he is the oldest and always has to do baby things because none of the other kids is old enough to do real grownup stuff.
I think it is very good that you respect his feelings, even when he is raging, and I hope your communication after the fits is equally respectful and loving. It sounds like you are a very kind and considerate parent, and that goes a long way toward having a kind and considerate child.