You have two options really:
being the "bad guy" but doing whats what is best for him and just saying no.
Personally you have your self and your younger son to create a home and a life for. Your older son (and gf) apparently think they are wise enough to make their own decisions. Now that your son is stuck in the illness he has now your help can only go so far. It is personal growth that will change him. Honestly allowing him to come and go as he pleases will not make him quit.
Allowing him to come and go as he pleases will only give him more breathing room and leniance almost as if you are saying his behavior is okay.
or
letting him do whatever it is that he does and control yours and your family's happiness and comfort.
Take charge of your life dont let your son have access to that.
