My brother ran away

My 15 year old baby brother ran away from home last night. He just got on his dirt bike and left the house. Don't really know what to do about any of this, my parents are trying not to freak out but still... They aren't calling the police or anything because my other brother did this at his age. They don't like having to follow the rules my parents set. Right now its a timed thing to see if he comes home or gets picked up for doing someing stupid. My mom and dad think he may be suffering from Bipolar, and that might be why some of his acting out makes little sense but now he's running around and not coming home. So now what????????????????????????............................................................................