My little brother is playing gangster?

He is 16 years old and has been acting and dressing like a gangster. He has been staying out real late and has been getting into trouble with the police. Well the other day he drove me somewhere (my car is in the shop) and he had to get gas. While he was in the gas station I dropped something and when I went down to pick it up, I felt something cold. I grabbed it and it was a gun. I got scared and put it back where it was (under my seat) and I never asked him about it. Should I ask him about it? How can I get him to stop doing this? Please help :(