ETA after your SWH: Thank you for sharing this update! So often, after thoughtful, but difficult responses/advice to hear, posters get mad, make excuses, and just leave. It is nice to know that you took the advice to heart and made something happen. You seem to be the only adult in this boy's life who actively parents him on a consistent basis. It is a great step that you and your husband are seeking legal control of your step-son's medical concerns.
It's probably going to be a fight, but make sure you have all the documentation from the school regarding his behavior, all of the phone calls from the school, detention records, notes from the social worker available when bio mom tries to argue against him getting treatment. Any reasonable person can see this child is headed for trouble without significant interventions.
I just want to point out one more thing. Your husband is abdicating his parental responsibilities by refusing to appropriately engage with and discipline his son, and this, as well as your step-son's behavior, must be addressed. You can't just drop your step-son at the therapist's doorstep and expect the therapist to "fix" him. It's the family dynamic that also needs to heal.
If your husband truly cares for his child and wants his son's behavior to improve, he has to make an investment of his time with this child. I know you do things on Sunday as a family, but this boy needs his father's time and attention and his own special time with his father. There should be regular times where he has all of his father's undivided attention. Even in families with super-packed schedules, involved parents make time for their children. Doesn't have to be hours at a time. Just regularly scheduled time where it is just the two of them.
You say your husband has the kind of job where he can't take off to go to school, and I have to call him out on that. If your child ends up in juvenile court, will he take time off then? Is that what it's going to take?
You say you take a few vacations a year. I'd suggest if there is no other way, that your husband use some of that vacation time to attend meetings with the school counselor and/or social worker. In addition, once your step-son starts treatment, it won't be just about him but also your family dynamic. Your husband should be prepared to attend those sessions as needed or requested by the therapist.
I really, really commend you for not giving up, for putting this boy's needs front and center, and for insisting that your husband get on board with getting treatment for your step-son. It isn't easy being a parent, and step-parenting brings its own unique challenges. Hang in there, and make sure this child knows that you love him and that you'll never give up on him.
Best to all of you.
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ORIGINAL:
Not sure what you mean by he has become violent with your other son, but you kind of lost me there.
Are you talking about typical sibling fights, and you are so angry with this child that you are calling it "violence" to make your case, or did your step-son actually attack one of your other children? If it was an attack, why would this be tolerated?
From what you describe here, this isn't going to sort itself out with time. This is definitely a situation where professional help is needed. It's all too emotionally loaded, with blaming, and denying, and a great deal of animosity, discord, and tension in the household. I can't imagine anyone in this home is happy.
Your step-son is on a self-destructive path, and if you and his dad, (and hopefully bio mom) don't get him some help, the outside world will force your hand eventually---whether it's the school, or worse, the criminal justice system.
You must search your heart, however, and be the bigger person. You need to be clear in your own mind, despite your frustration, that you must be there for this child to try and help him. Right now, you're seeing him as an obstacle to your happiness rather than your stepson who is in need of help.
I'm not saying that what he is doing is okay. Lying, stealing, and disrespect are not okay. But here's the thing: he's the child. You and your husband are the grown-ups, and you both, together, need to act. This cannot be allowed to go on. You have other children who are also being affected by this, and your family dynamic is becoming more and more unhealthy.
Start looking for experienced, licensed family therapists. Call your insurance company to see who is on the provider list in your area. Don't wait. It's gone on too long, and everyone in the home is suffering.