Unfortunately being bullied is all too commone these days. I think the main reason bullies bully is that they lack self esteem. They are afraid of anyone different then themselves.
My granddaughter's school had a series of classes on what to do to prevent or stop bullying. I wasn't able to attend.
I agree that the parent of the one being bullied needs to stand up for the kid when the kid isn't able to stop it themselves. This means going to the school, talking with the teacher, principal, and/or counselor if there is one. If possible arrange to talk with the parents of the bully. You may not get help from the parents. Instead you'll better understand why their kid is a bully.
My daughter graduated with a young man who has since become a scientist. He very much looked and acted like a nerd. He was painfully shy with most of his peers. I don't know if he was bullied. He never said. My daughter became his friend.
My daughter is outgoing and one who stands up for herself and others. Your sons should look for this sort of person to be their friend. My daughter met him in one of the high school clubs.
I suggest trying to encourage your boys to become active in activities in which they're interested. This way they can develop their own self-esteem as well as meet other kids with the same interests as themselves.
Help them to focus on making good friends instead of trying to defend themselves to the bullies. I do believe that the bullies are in the minority and they choose to pick on kids who are not only different but who are also shy and show that they're upset when bullied.
My daughter was having difficulty with a boy in middle school who was a bully. Her Dad said that the next time he tries to pick a physical fight to slug him right back. I'm not, in general, in favor of physical force. However this worked for my daughter. She did get a suspension because the school had the policy that in a fight both would be suspended. She was angry because she was suspended until she went back to school and this boy no longer baited her.
Her situation was a boy who was a physical bully. The verbal ones she had already learned to handle by either not responding (i.e. act as if she didn't hear them) or responded with a confident statement such as "so what." She did get into some verbal fights but she is very strong willed and wouldn't bend for anyone.
I doubt that a verbal argument benefits anyone. I would suggest that the best approach would to walk away in a confident manner. "Fake it until you make it."
Having friends and being involved in fun activities makes the bullying seem less serious.
If and when the bullying does become serious such as making threats or wanting to physically fight I recommend that both you and your son go to the principal and tell them what is happening.
At my granddaughter's school they had several 6th grade bullies on the playground. The counselor organized active games such as basketball and a teacher was assigned to the part of the playground on which the 6th graders played. I am a recess volunteer and saw that keeping the kids active cut back the opportunities to bully.
I realize that my granddaughter's school has always stressed respect. It was my daughter's school too. Not all schools have been able to act in such a manner that the majority of the kids respect the teachers. Or are in neighborhoods where parents also respect the school. This might not work at your son's school. I suggest that you volunteer at the school and keep in contact with your son's teachers so that you do know what is going on. You show your sons how to be assertive and not aggressive.
This may be difficult for you, depending on your personality and experience. Perhaps you could get together with other mothers so that you have support. I don't know if the PTA would be helpful. PTAs are sometimes more focused on being their own social group instead of recognizing that all parents have something to contribute. You can check it out.
There are some good books on bullying. You may find that reading one of those would be helpful in showing you how to help your sons.
I think that building self-esteem is the key to ending bullying. That is not so easy to obtain. I have read some parenting books that told of ways to help build your child's self-esteem.
I wish you well in doing this. Confidence is not easily learned either. I have frequently "faked it until I made it." I started out as a shy woman when I joined the Sheriff's office. No one would believe it now.