You and all family members (step and natural), including extended family, must agree to a zero tolerance policy for:
Physical abuse
Stealing - prescription drugs, money, credit cards, valuables, electronics, etc.
Property Damage
Illegal Use of both illegal and prescription drugs
Fire setting
Injury to animals
Threats to hurt people or to steal or damage property
This means that you call the police and file charges against him every single time he commits one of these offenses. You need to have a clear plan for when and how you will call for help so that everyone uses the same criteria when possible. Inform step-son of the rules and the consequences for breaking the rules. Then you MUST enforce the rules, with outside help if necessary.
We have an action plan posted by the phone in plain sight. All the professionals we work with (psychiatrist, therapist, etc) have a copy of it for their files so they know what the plan is and when it will be activated.
If he threatens to hurt you then call 911 immediately. Do not wait to see if he really will hurt you. If he actually assaults you (shoving, kicking, hitting, slapping, etc) get to a safe place if possible before calling 911. When officers arrive you must agree to charge him with a crime and then insist that he be removed from the home for your safety. If necessary carry your cell phone on your person so that you can make that call immediately without having to get to the house phone. Be prepared to leave the house to make that call or to lock him out if possible. If necessary lock yourself in a bathroom.
If there are siblings, especially younger children, a plan must be in place for them to also have a safe place to go and they must know when to go there. If possible they need access to a phone to call 911.
If you have called officers out and failed to press charges in the past then you can expect they will be resistant to responding in the future. If that is the case, I suggest speaking to your area's community policing officer (if there is one) or call the police department and ask to speak to someone about the problems you are having with your son. In that conversation you need to make it clear that you are now willing to press charges against him whenever you request assistance.
You should already have taken many practical measures like locking up all financial papers, wallets, purses, valuables, prescription and non-prescription medications; installing a locking mail box that only you can access; installing a dead-bolt on your own bedroom that only you and husband have a key to and then locking your door at all times; locking up all individual cell phones at night in your room with you - do not leave them out in the house charging; changing the locks on the house so that he has a key to only one door then you can be sure that he can't get in through other doors if you have locked him out; refusing to provide cell phone, car, bus pass, allowance, computer, video games, handheld games, special clothing, movies, bike, skateboard, anything that would normally be earned by good behavior.
You should be inspecting his room and his belongings like backpacks without notice. You may want to consider removing the door to his room as a visible reminder of where the authority rests in the home and that he has to earn privacy through good behavior.
When inspecting his room and belongings, if you find drugs, drug paraphernalia, stolen items including prescription medications, illegal weapons of any kind (switch blade for example) you need to call the police immediately and turn him in. Do not remove the items or disturb them if you can avoid it. Just leave them in place and call the police.
If you have good reason to believe that any of these measures will provoke a physical attack, then you may not want or be able to do them. Or you may need to carefully choose your time. Inspection of his room and belongings can be done when he is gone. If he will know when he gets back that you've been in his room then you should be prepared for a violent response. If you found drugs, etc and reported him to the police in his absence, ask them for advice on how to handle his return home if they do not locate him first. If you found weapons, especially guns, do not stay at the house until the police have found him and told you that he is in custody.
Placement in a treatment facility for dual-diagnosed youth would be great but these are few and far between, especially if you do not live in an urban area, and as others have said private programs cost a fortune. It also requires his cooperation to some extent. He may have to spend some time in juvenile hall before you get this, and you may not get it even then. I personally would not expect him to cooperate.
If he has figured out how to work the system and feels no remorse for his actions it may be too late to help him and you must turn your efforts to protecting other family members physically, emotionally and financially. If there are younger siblings, it is vital that you get him out of your home in my humble opinion.
You probably need to consult a lawyer now about what to do if you need to legally transfer his custody to the state or his mother in order to get him out of your home. You may also want to ask about restraining orders at the same time.
Sorry to be pessimistic but in our experience, once a kid has done what you are describing and either gotten away with it or persists despite treatment and/or big consequences it is generally too late to get them to turn back. They must hit bottom and decide for themselves that they don't want to live as a drug-addict on the streets for the rest of what will be a short and painful life. He will not believe that this is what is going to happen and you cannot tell him or show him. And he may not ever get there, especially if he's gotten involved with a gang.
If step or extended family members refuse to adhere to the plan then you will have to decide how to handle that. He must not be allowed to split the family, pitting one person or group against another. I'm guessing that is a big part of how things got to where they are now. If there is agreement between the adults in the home when he is living there (for example Dad and step-mom) and extended or step family do not agree then I would inform them of the reasons for the rules, the nature of the rules and what you plan to do when he breaks the rules. If they feel that you are exaggerating or playing favorites, then I would insist that step-son goes to live with those people and not stay in your home. Visits would take place away from home in public places.
If you and Dad do not agree about the rules, calling the police and pressing charges then you are probably going to have to leave and take any younger children with you, at least temporarily. Ideally you (or others) will have pressed charges against step-son at least once before you have to leave, thus ensuring an official record of the lack of safety in the home.
If Dad is also abusive in any way (verbally, emotionally, financially, physically) then you need to call your local domestic violence hotline and ask for help tonight.
I realize it's likely to be complicated but you must find a way to set concrete limits on your step-son and enforce them. If he can just run away to Mom's when he screws up at your house then things will only get worse. In that case, he needs to live at Mom's and let her deal with the consequences of her choices. Painful as it may be, Dad may need to give up custody or take other measures to ensure that his current family is safe, assuming you have other children.
Most of the folks on the board understand the danger your step-son represents to himself and to his family. We take that danger seriously and will not poo-poo your concerns or reports of problems. Many of us have experienced these kinds of problems and know that safety of all family members is the most important priority. We know that you are likely to be isolated and to have few if any people to turn to for help or who understand what you're going through.
Step-son may have snowed everyone else well enough that you are not believed and are even accused of victimizing him. Many abusers are very clever at hiding their behavior from well-meaning people. We believe you. You are not crazy. If necessary you may need to set up cameras in the home to tape him in action. Hopefully not but you must do whatever it takes to keep yourself and any other children safe from him.