As a kid who was bullied all through middle school I can give you a little bit of advice.
(though I'm a girl)
Honestly, I wasn't helped by ignoring them. It made no difference to THEM and they never got tired of making me feel utterly worthless. My sweet loving parents did everything they could to make me feel supported and encouraged and while it did make a difference about who I am today, I've had to have years of therapy to try to get over my bad opinion of myself. Much less, I developed a deep desire to be accepted (because I wasn't).
In eighth grade I was sexually harassed by a few of the boys in my school. It got so bad that I would sob in the car begging not to go to school. Once I was finally forced to tell my parents what was going on (I was too embarrassed to tell them sooner) they set up a meeting with the principal and my homeroom teacher in order to have the situation taken care of.
The best thing you can do here is get involved. Those kids should not be discussing gang things at school, it's just as inappropriate as sexual harrassment. I would go so far as to say those children should be suspended for even mentioning a gang on school property (especially with how young school shooters are becoming these days). I just don't believe bullying should be ignored.
The principal should know what is happening and his teachers too. Not to mention the parents of the other kids.
I do very much agree about Tae Kwon Do.
I took that as a child and it gave me something to feel proud of myself for. Learning his heritage will also give him something to make him feel proud of. Part of why I made it through school was my artwork. It gave me something to feel confident about.
Best of luck with your GS, I feel for him so much. Dealing with bullying is very difficult.
I'm so sorry my post isn't more encouraging or uplifting! X( My life got SO much better in high school, I had great friends and I loved going to school to see them. My parent's support really helped, too.