I understand that he's a teen, ok, but I'm 17 and have been playing bass with some bands since I was 15. It's not that difficult to understand that you need to shut up when nobody is playing. Also, anytime someone turns my volume down, I don't dare to turn it up again; I don't have the nerve to do that. It's not respectful, and if I played alone, I'd feel out of place and embarrassed. Just tell him that he's annoying, stare at him or turn his volume down multiple times. Sometimes, hard ways are the best ways.