Essentially, at 14, your step son would have a very strong 'voice' in deciding where he wants to live. But, at this point in time, only the people with parental responsibility for him can provide a roof over his head, unless they agree to let him live somewhere else.
As you don't have parental responsibility, you'd have to apply to court and you would need to ask for permission to make that application. In reality, of course you'll be granted permission, you have cared for him for years. But it's little hurdle that will add delay to the process.
The court would then look at his best interests and his views in making a decision about where he would live.
Have you spoken with his father and tried to see if you can come to some form of arrangement whereby he stays with you in the week, stays at his current school and goes to his dad's for the weekend?
You can apply to court of course, and with his view being that he very much wants to live back with you and his siblings, it is the likely outcome - it may just take time, that is all.
So a compromise in the meantime may be worth trying to explore.
You also have to go through the hurdle of mediation (or at least attending s mediation assessment appointment - MIAM) before applying to court - another hurdle and another delay I'm afraid.
To set up a MIAM, you can contact a mediator here: https://www.familymediationcouncil.org.uk/
The c100 form for the court application is here, and the application costs £215: https://www.gov.uk/government/publications/form-c100-application-under-the-children-act-1989-for-a-child-arrangements-prohibited-steps-specific-issue-section-8-order-or-to-vary-or-discharge
I hope that this assists and answers your questions? Can I clarify anything further for you today?
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