If I were you I would probably go down to the police and ask what the consequences would be if he were at your house and you called the police. Would he go to prison or not, and for how long? How would he be cared for in prison to make sure he was not preyed on by other inmates? What would they do about his drug use?
I would want to know the reality of the situation, and not the rumors that could happen. I would want to know how his detox would be handled and how they could keep a vulnerable person from being preyed on by other inmates when the vulnerable person probably would not be able to realize that he is being taken advantage of. Your son is NOT the first person to be in this situation.
Once I knew more about the situation, and what was likely to happen, I might be more likely to let him end up in prison. No, prison isn't nice. However, you would know where he was, that someone was looking after him, and that his access to drugs was limited. Of course he would tell you stories that if you didn't smuggle this or that in, or put money on his account, he would be beaten up because he owed money for drugs. Report this to the authorities every time. Make sure he knows ahead of time that you will do this. Be very clear that you will NOT be party to any schemes to smuggle in drugs or to buy drugs in prison.
It might be a way to get him off of drugs for a certain amount of time. Of course he would have to want to stay off of them for it to have any long term impact, but it might be a break for you.
Are you aware that every dollar in food that you give him is one more dollar in drugs that he can use? If you buy him food every day, what are his benefits supposed to pay for? He isn't using them for rent. He isn't using them for food. You pay those things. So his benefit money is drug money, along with any other cash he can get from you. Or anything else he can get from you that he can sell. It is good that you are not giving him cash very often, if at all. Now it is time to see the food that you are giving him as just another resource that is freeing up his resources so that he can buy drugs. The less you give an addict, the harder their life is. The harder their life is, the sooner they want to quit, hopefully.
I know it is hard. Sadly, providing for all of his needs hasn't helped him break this addiction. It has only contributed to his bad behavior, deplorable treatment of you, your husband and of his sister. You know how to tell when he is lying because it is when his lips are moving. There is literally nothing that he says to you that is the truth.
It would be impossible to cut him off cold turkey, or to stop worrying and obsessing over him cold turkey. He is addicted to drugs and you are codependent with him. Addiction is the disease that infects the entire family even if only one member is using. Trust me, I know. My grandfather was an alcoholic who never drank around me. I learned all the behaviors of the grandchild of an alcoholic. My brother is an alcoholic, thankfully in recovery. Your ENTIRE family needs help, not just your son. You cannot get your son to go get help tomorrow. You can get yourself to get help this week, or even tomorrow. You are already getting help here.
Go to AlAnon or NarAnon meetings. They are 12 step meetings for families of alcoholics or drug addicts. Here in the US, no one minds if you go to AlAnon but your family member is a drug user, or you go to NarAnon and your family member is an alcoholic. At least they don't at any meetings I have gone to. You can google whichever one you are interested in and your area to find meetings. There might be more of one or the other, which is why I suggested both.
Private therapy is also helpful. Look for a therapist who is experienced with substance abuse or addiction and/or codependency issues. It may take a few tries to find the right therapist. If you don't feel the therapist is right for you, don't go back. Find another one. There is NOTHING wrong with going to another therapist. Many of us have come across therapists who gave us advice that was wrong, idiotic or crazy. Always use your common sense and follow your instincts. If you are in doubt about something a therapist says, you can always ask one of us here. If you don't want to bring it up on the open forum, a private conversation between you and the person you want to speak to is easy to do. It is important that you find the right fit with your therapist, even if you have to see several before you find the right one.
There are also books that are helpful. Codependent No More by Beattie is a classic for a reason. Boundaries by Cloud and Townsend is incredibly helpful, especially with the workbook. There are others, but these are a great start.
I do advise telling your son that you won't tolerate his abusive language over the phone. That you will hang up if he is rude or he swears at you. Only do this if you can truly hang up on him, though. If you tell him this, and hang up on him when he is rude, you might start teaching him that you have some boundaries that he would do well to follow. It is just a thought.
(((((hugs)))))