Honestly, I feel you might be overestimating the advantages of a move to the UK for your stepson. Leaving aside the issue of schooling, and the difficulty he might encounter by having 'nonstandard' qualifications when applying for jobs, internships, etc., those opportunities are in fairly short supply. While, from an 'earn while you learn' vs 'pay tuition' standpoint it might seem preferable to have him train in the UK, getting the internship isn't as easy as registering at the local Community College.
There's also the issue of portability in qualifications. A City & Guilds or NVQ isn't going to mean much if he goes back to the US. His working experience would help, but he'd still be missing out on, for example, certain manufacturer certifications.
And then, of course, there's the happiness factor. Some people are more adventurous than others, some are more adaptable, or resilient, or whatever. That's great, but I think it's important to try to avoid thinking of people who aren't , as somehow flawed, and to think they can be 'fixed' if they just try hard enough.
It sounds as though this young man has already had to deal with moving recently, and having his life uprooted at a very delicate time in his life. This may simply be too much for him to deal with, and, as Becca was saying, a set-back at this stage of his life-- whether emotional/social or educational/vocational, could have a lifelong impact.
So, provided he's given all the info, on all his options (including what happens once he's 18-- remember, under-25s are excluded from most UK benefits, including housing benefit, and under-21s are often paid at a lower wage, so he may be stuck at home with you for years), and provided you and your partner are confident that his other family members can be relied upon to provided a safe home for as long as he needs one, then I would urge you to respect his wishes and let him stay in TN.
Immigration can be tough on anyone, even with the best intentions and circumstances. Forcing it on an uncooperative teenager is unlikely to end well.
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