School dilemma - would you change a child for Y6?

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I am currently torturing myself trying to decide what to do for my DS, currently in Y5. My kids are in a fairly good international British school in South East Asia. We arrived here in January 2013 so 18 months ago.
We just got the notification that our son is accepted in an excellent IB school here for september. The trick is that we are very likely to leave at the end of next year (june 2015) and will probably go to the US.
If we stayed longer, I would not hesitate to change him, but is it worth it just for a year? For context, most of his friends are leaving his current school as it is not good for middle school and I don't even know how many Y6 classes they'll have. At the same time, I have been fairly happy with the school so far. My son is a good pupil, not excellent, but doing well.
For those of you who are familiar with both British curriculum and IB, do you think it might make a big difference for him? Does it make more sense to finish his primary years in a consistent system?
We are hoping to find an IB school once in the US, but we might have to go to an American one (not that many options where we are looking).
I would love some advice!

I personally wouldn't if I thought my DS was making progress in his existing school. It is a big regret of mine that my DC have moved school so many times as I can see gaps in their education . If you can reduce this by keeping him in this school for a year I would do so . With the caveat that your DS is happy there, if not then I would consider moving

Thanks for the answer! This is my gut feeling but I keep getting comments on how good this new school is, which make me doubt...

Yes but even if it is better you are unlikely to benefit that much in a year. It probably will take your DS half a term or so to settle in. Also a school is generally only as good as the teacher . Do you know which teacher your DS is getting next year? have they got a good reputation?
Also buying another uniform just for the year and all the hassle of getting to know a new school . I wouldn't

Agree with all the above comments. I would stay put and learn as much about the school in your new location to help him transition into middle school.
IB does not have a great reputation for the MYP. Coming from the British system, he will be in a strong academic position and will use that first year to establish himself securely without too much pressure. He will also have good literacy and numeracy foundations and can concentrate of acquiring the 5 pillar mentality before it switches full gear after grade 9.
I only know US schools overseas which to be honest tend to loosely follow the IB framework to keep up with the rankings.
They usually perform very well in SATS if further education in the US is a consideration.

Thanks both of you! I declined the spot today, I hope I won't regret it, but I feel it is best to finish his primary in a consistent setting. (Also will avoid us having to pay double for one term and having inconsistent term dates for our three children, on a more practical note.)
Just need to look into schools and curriculums for middle school now!
We are actually French but have been abroad for a while now and are unlikely to go back to France any time soon. I find education systems to be very hard to understand and compare... One of the most challenging aspects of being expats, I feel. Thanks again for your input.