It's a psychological addiction
Though his use of marijuana has proven to be detrimental to many facets of your and his life..the good thing is that research shows people do not become physically dependent on marijuana. It is not the same as with opiates and alcohol, for example.
The first step (easier said than done) is for his true desire to stop to surface and then to remove himself from all things/people associated with the drug. I had to do a debate in college on whether or not pot should be legalized and there were many more pros than cons to it being legalized.
Additionally, the whole gateway drug theory is simply a myth.
Try to be diligent but patient while helping him get to the root of the problem that is causing him to use it as an escape in the first place. And no matter what, do not blame yourself for his lack of progress, relapse, etc.
Most of the time, good ole intuition will show us when the bad in the relationship outweighs the good to the magnitude you can no longer tolerate, it may be time to separate, help as a friend only, or exit the relationship to save yourself.. if necessary
*First post, so I apologize if it is not clear and concise*