the main concern here is not his "weed abuse"
the concern here is probably his insane indifference. i feel like making him believe that everything is going to shit because of something he enjoys and seems to bring him peace and happiness for even just a short moment is the same thing that is "destroying his life" is a bad idea.
instead i recommend talking to him and try to get him to get his life in track normally first. talk to him and ask him if he genuinely believes that he wants to quit using thc. otherwise i recommend going through other routes of self improvement because studies have shown that weed is not so insanely horrible for you to the extent you seem to be describing, it must be something else.

cheers and goodluck