The problem with fighting back is many schools have a zero tolerance policy. Both kids are suspended or expelled for fighting. Some kids (like me when I was in elementary and high school) followed rules to the letter. It is easy to say "beat the snot out of the other kid" but some kids don't want to get in trouble for fighting. Getting into trouble can cause more fear and anxiety than the bullying. My bullying in high school only stopped when I managed to get my tormenters expelled.
Schools are a place where a child should feel safe. Is it safe if a child is bullied? Absolutely not.
I will also tell you a small story about my uncle. He was a boy like your son. Not into physical sports, he'd rather pursue something intellectual than physical. This did not sit well with his father, my grandfather. My grandfather was aggressive that his son wouldn't be a wimpy pushover. Because my uncle did not like sports, this somehow made him inferior in my grandfather's eyes. My uncle wouldn't fight back against bullies, either. It just wasn't his nature. He grew up to become a brilliant veterinarian (he even worked with Dolly the cloned sheep) and teach at a major veterinary school. But he harbored a lifelong resentment to my grandfather for pushing him to be aggressive when it wasn't in his nature. Truly, as an adult my uncle was just like when he was a boy, a very warm, kind, and gentle man. He also resented his mother, because she sat back and did nothing while her husband did this to him.
The moral of my story? I guess not everyone out there is a fighter. Trying to make someone fight when it isn't their nature does more harm than good.