OP Please don't take too much notice of the people here who have had no experience of having a child board from 8. I have and I'm going to try to be constructive.
Of course boarding is not for every child; but you do have to give it a couple of terms. My son was miserable for the first term and a half but then gradually got better and by the beginning of his second year really loved it ("Thank you so much Mummy for sending me to xx school- I really love it now"). It is absolutely ghastly going through that initial stage, especially if you are abroad but you are probably seeing/hearing the worst of it. Most of the time when he is not with you or talking to youhe will be busy and happy making new friends and doing new things.
If after giving it a good shot he really is unhappy boarding it is not the end of the world for entry into the competitive schools- the pre-tests are not taught for and it is only after gaining a place that a "top" prep is useful. By contrast, my boy was at such a school from the outset and failed almost every available pre-test.
School entry notwithstanding, I would still have sent him to his boarding prep because he had an excellent education and an absolutely idyllic few years. I would also still have sent him at 8, so that he could make the strongest prossible group of friends and really settle into the culture. The difficult first term has to be gone through at some point.
If he seems miserable or worried, then do talk to the staff responsible for him. As Gruach says, the level of very genuine care in these schools is extraordinary. Enormous care is taken to help boys through the hard patches. I wonder when you say that the problem is "negativity", whether your son is just really homesick and struggling to express why he is unhappy. All boys struggle a bit with getting themselves organized when they first start and will be chivvied along to find the right socks etc.; but this is generally very gentle. Be straight with the school if this is upsetting your son and, with an eight year-old they will probably take a different approach. If the problem is straight homesickness they will have seen it all before and be very good at dealing with it- a mixture of dogwalking, lavender oil and playing in the snow seems to have helped my son.
It is harder for those whose parents are abroad, especially when they see other boys' parents at the school so frequently so you may find yourslef here for a few more exeats etc than you had originally envisaged; but it will be worth it if you can help him work through the settling in period. Also, it's a good idea to try to make sure that he has a letter or card or little parcel from you a couple of times a week.
I hope that you haven't been alarmed by some of the posts on this thread. In a term or two he will probably have come to adore his little world and many of the people in it. When my son left his prep at 13 there were at least as many tears (boys and mothers) as there were on the first day. My son's prep was basically a paradise for little boys where they learned quite a lot into the bargain and made fantastic friendships. Don't panic and give up too soon.