How can I get my brother to study harder, and understand that his grades are important?

My brother, who is a junior in High School, is slacking on his grades, he has low B’s and C’s. He is taking the same courses as I, rather rigorous classes.

I KNOW he is really smart, and the times he puts his mind to it, he can do an AMAZING job. But, the problem is, he doesnt put his heart on mind to something much. My parents have tried various ways to tell him to get his grades up. I have tried my ways. He doesnt listen, and continues to mess around on facebook, social websites, and games.

How do I persuade him to understand the importance they are for his future life? Or, even motivate him?!