Honestly, I don't know the secondary system well. Hopefully ksand will let you know what it would mean to enter in Year 9 v Year 10. Ksand is British and a huge success story out of the UK school system.
Aww, thanks
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Honestly, the important thing is to make sure he's in the UK in order to start Year 10, so that he can begin GCSEs at the correct time (i.e. you don't want to move him AFTER Year 10 starts).
I'm not sure if it's changed since I was in school (it's been 20 years since I started Year 10!), but the only thing you really did in Year 9 that had any affect on Year 10 was to choose which subjects you wanted to take for GCSEs, and for the teachers to decide which 'sets' to put you in for some of the subjects based on your Year 9 grades/performance.
It used to be that you would sit SATs (Statutory Assessment Tests) in English, Maths and Science at the end of Year 9 (in May), and your scores were partly used to determine which sets you would be put in. However, SATs for 14-year-olds (Year 9) were scrapped in 2008 and are now only taken at age 7 and age 11.
The actual work you do in Year 9 isn't really related to Year 10, since GCSE's are self-contained 2-year courses, which you start at the beginning of Year 10 and finish at the end of Year 11. So, as long as he has the basic backgrounds in the subjects he's taking for GCSEs, that shouldn't be an issue.
However, if you wanted to give him some time to adjust to the UK school system, make some friends, and allow the teachers to get to know him and gauge his academic ability, before GCSEs start, it would be worth him starting in Year 9 this coming year.