I've seen this kind of thing over and over and over in my community.....
Truth is, nobody can make anyone else stop. You can spend your life's savings on the best care out there, and it wont do jack unless he actually wants to quit the stuff.
I hope he sees the light soon, I hope he can look at himself and his failing classes and want more for himself.
I smoked a boat load of pot during HS, but I never came home stoned off my a.s.s and my grades never suffered. I remained one of the top students, so guess that's why nobody suspected I was doing anything wrong in my free time....
Your son sounds like he's on a different level all together though, its long past harmless experimentation and enjoyment if he's failing and acts like he doesn't have a care in the world.
I really hope he wants better for himself.