I know I would be very uncomfortable knowing this was in my house so protect yourself please.
Is his dad someone you can talk to? Is he someone your son would talk to?
Because he might be a good buffer right now.
I know as a single mom it can be scary to confront our sons.
If his dad confronted him about it while your son is away from your house but knowing you have the evidence, his reaction and response might help you decide what you'll need to do next, and it might get dad to wise up to it also.
Only you know if dad would be any help, but I think he needs to know.
Most cities have gang intervention units that can give help to you and your son. I don't know what city you are in but do an Internet search.
I agree that the school might be a good place to start, they might have resources for you.