Seriously, Rebecca, get out NOW, before he realizes that this is for real. Or, at the very least, get your younger son out. Speed freaks can be extremely violent, and it sounds to me as if, given his total lack of hesitation to mentally abuse you, your elder son wouldn’t hesitate to hurt you or at the very least, steal everything salable in the house and sell it. Right now he apparently still believes that you can be manipulated. Worst case, let him think that he has succeeded until moving day, and then have your SO around armed. He could seriously hurt or kill you or your younger son long before the police could arrive to stop him, and in the meantime he could destroy or sell off virtually everything you own.
At this point, the person you are dealing with is not your son. He is a totally different person who happens to look a lot like your son and knows how to push your buttons. He is extremely dangerous to your property, yourself, and your younger son. He is quite likely also a danger to the neighborhood at large.
Someday you may get your son back. You also may not. My borther just lost his partner of two year, who had been addicted to heroin, but (as far as my brother could tell, and he was controlling things pretty strictly), was not using now. He didn’t directly die of a drug overdose, but his long years of abusing many drugs weakened his body so that he died in his mid-to-late twenties due to a highly serious case of bronchitis. J. had straightened out in many ways - he had a job, was doing well, had hugely improved relations with his family, but the damage had already been done. This was the second time my brother had been “widowed,” the first being when his lover of at least 15 years died of AIDS in the late '80s, and he is suffering terribly, especially since he was here with his original family (parents and siblings) rather than at home when it happened (he got the call on his cell phone on his way to the airport). At least the autopsy report eventually relieved him of the fear that J. had taken advantage of his four day absence to start using again and had OD’d. My point here is that even if your son straightens himself out (something you can never do for him), he’s not out of the woods entirely; there’s simply never a guarantee of a happy outcome.
Bad things can happen to really, really good people, and it’s terrible when they do. But life is not fair: it’s a crap shoot, and sooner or later the house will always win - hopefully painlessly after a long life. You have good reason to hope - your son is young, presumably fairly healthy, and may yet recover and live a long and prosperous life. Or he may get run over by a bus tomorrow.
One thing you can rest assured of; if he does recover from his addiction, he will not blame you for ‘deserting’ him. He will probably thank you. Because he will have become your son again, rather than some psychotic stranger living in your son’s body.
But in the meantime, please take every possible precaution - most importantly getting your younger son OUT of the house so that he will neither be hurt himself nor have to see you getting hurt.