Start with the immediate resources available to you: your stepson's pediatrician, the police (ask for the school resource officer who deals with teens), the school counselors (if anyone is still there after the close of the school year). Your town may have a department of children's & family services. All will know about what therapeutic options are available to you at a price you can afford or which are covered by your medical insurance, or a combination. Don't start with trying to choose a program (such as "juvie" or an inpatient rehab) - you don't know how to choose yet and there are multiple issues at play (drugs, sex, rebellion, defiance, etc.). But you must do something quickly because of your stepson's behavior and its effect on him as well as the other children. This is a job for the child's parents unless you have custody and the mother is not involved legally - you as the stepmother can start to research, but the child's father and mother have to make the decisions.
You have discovered that you can say things to a teen but if there's no follow-through, nothing happens. You've made rules, he breaks them. You've told him he can't live there, but you can't/won't kick him out. I agree you have to find a facility for him, not just boot him out, since he is a minor. But he is actively defying you. He's showing his immaturity by saying he's going to call the police on you. I'd say, "Great. Let's call now and have them send someone over." But he may have gone too far for this to be effective.
There are plenty of services available to him. You can also see about family support services because everyone needs help with this - establishing boundaries, protecting the household. You can try one of the family services such as Al-Anon or a parallel program for drugs vs. alcohol. But the strategies are similar so get to the one you can easily attend. There are also on-line resources. Any family therapist to whom your doctor or the family service people refer you will be able to help you make connections.
For now, understand that an out of control teen is screaming for help, and that continuing to allow this behavior actually amounts to a form of parental neglect, not just of the other kids, but of this boy. Reach out today, take good notes with everyone you speak to, and start putting together a plan with people who know how to do this.