Roommate doesn't ever lock door.. need to talk

So I live in a bad neighborhood. there's shootings that happen right down the way few blocks over all the time. My room mate is my cousin he's only 19, but he never locks the door anymore unless he's leaving for a day and won't be back that night or if no one's here. He doesn't lock it because he's use to living in good neighborhoods. He's even been locked out of the apartment twice because he doesn't bring his key. (I figured he would've gotten the hint but it's getting worse)

But anyway my boyfriend's son stays over sometimes and he's 2 so he gets into everything. me and my bf are both getting to the point it's scarry for me and his son when my roommate doesn't lock the door. I'm already scared to be home alone, it's worse when I know the door isn't locked.

I need to say something, how do I put it without being rude?