K.S.
Don't be naive to believe that gangs are driven by city boundaries versus suburban boundaries. They are in Oak Lawn as well as drugsthe drugs are. Oak Lawn HS has been known for having both of these problems since I was young.
That set aside, I would set up a meeting with the principal, school counselor and teachers to discuss this problem. All schools have things in place to to deal with bullying. If they're unaware as you were previously then they can set up a plan. If the plan is unsatisfactory to you pull him out. His physical and emotional safety is paramount to anything else.
Most important in all of this for you is going to be your response from here on out. You must present a calm, loving, supportive and confident demeanor. You will find a way through this. Best wishes.