Difficult 18 year old.

I love my son, I really do, but no matter how much I try and advise him, he really doesn't listen. He's a senior, won't put effort into his grades, believe me I've worked with his principal, teacher's and him. He just doesn't care, there's nothing more I'd love to see than to watch him walk across that stage, and I'm feeling doubtful. He could care less about anything other than hanging out with friends and messing around. Any advice on what to do? Believe me, we've tried so many things, grounding, taking his car away, everything and it always falls back to the same. I know he's of age, but I'm feeling so defeated here, I am at a loss for what to do and make him really see the importance of his actions and what he does.