My nine-years-old son is very peaceful. He hates violence, to the extent that he doesn't defend himself when other kids hit him at school -- although he's strong and can fight well. Do you consider this a problem? Am I supposed to guide him to do otherwise? In my day, I used to fight back without thinking of consequences.
Your son sounds like a great kid. If he's suffering from the mistreatment of others, the solutions are probably best worked out with school officials.
Your own experience of fighting without thinking of consequences is something you grew out of because you matured. Fighting can be counter-productive and even dangerous. The lessons you learned have made their way into your peaceful boy, and now the challenge is to protect him, and nurture that which is peaceful in him.