The boarding schools survivors association is entirely self selecting those who've boarded happily are obviously not going to participate. At DS's prep loads of the fathers were old boys and had full boarded from an early age, they obviously hadn't had a horrendous experience otherwise they wouldn't have chosen to send their own children. The idea that is frequently peddled on here that boarders grow into dysfunction adults unable to form meaningful relationships is also disproven by these fathers as like in all walks of life some were charming, many ordinary, some unpleasant, some simple weird, some were obviously very happily married, a few were divorced, no different from parents I've met at all the other schools my DS's have attended, day or boarding state or independent. The most charming delightful happy couple that we know (and go on holiday with) both boraded from 7, they have produced typical teenagers (who board). I meet lots of boarders many have boarded from a young age again generally most are well adjusted. I do meet the odd screwed up one, but I also meet children who've never even set foot in an independent school let alone a boarding school who are equally screwed up.
So I come back to my previous advise OP I think you need to look at the individual school, are you blowing out of proportion an off hand remark perhaps said in that hectic moment of picking up a child at exeat or is the school really expecting too much? Perhaps they're not as caring as you thought, although I would hope in 2016 that all boarding preps have outstanding pastoral care now, realistically they'll have too to survive. Would your DS be better off at home for perhaps another couple of years and if he gets a place at a UK boarding school coming here for his last two years of prep, or even joining a UK boarding school in yr 6 when most interviews for top name school take place if you feel they'll prepare him better than a school in the US? Or maybe keep him at home till yr 8 and see if he can sit a different entrance exam if his school won't prepare him for CE.
As peternas has said there are many excellent caring preps in the UK, many will now have lots of young overseas boarders and know how to care for them properly, maybe it would be worth your while just talking to a few I can suggest a couple and see if you get a better feeling there and it will give you something to compare your current prep with.
Don't be too swayed by these very emotional comments on here, many are died in the wool anti boarders most importantly with no current experience of boarding in 2016, only anecdotal stories stuff my friends DH's is a sociopath or my neighbours best friends brother had been divorced twice and cannot form any meaningful relationship type of stuff and they boarded. The anti boarding brigade will also even when presented with categorical evidence that a child who's boarding is happy and well adjusted won't accept it.
This is a frequent topic on MN and every time I always get at least three PMs from parents who's children are boarding and even boarders themselves who say they don't recognise the picture portraid by the anti boarders. peternas (extends an olive branch) and I have locked horns in the past but on this subject we are both in agreement and we are talking from a position of actual understanding and knowledge unlike many on here.