There are treatments far more effective than pot, but he refuses to avail himself of them.
Chicory, you could read a whole bunch of stories about parents who have kicked out their adult children, and some would have happy endings and some would not. There are no crystal balls and no guarantees.
His life with you isn't what most people would consider "living". It isn't like he is happy and healthy now. You have two miserable people, chained together by one person's refusal to get help for himself. How likely is it that he will get well by continuing on this way? If you kick him out, MAYBE things will get more unpleasant for him for awhile, but he at least has some motivation to change his life, which is something he doesn't have now. It may be his only shot at getting help for himself.