SaturdaySunday you cannot 'repeat a grade' in the UK state school system, you would have to go private for that. Which school year your child goes into is far more rigid in the UK than in the US, you have to stick to the correct school year to your age by law unless there are exceptional circumstances- by these I mean SEN/disabilities affecting education.
For example, my DFD is 16, born April 1997. In September 2001, when the school year starts, she would have been 5 by 31st August 2002, so she started reception that year, along with all other children who turned 5 between 1st September 2001 and 31st August 2002. Year 1 is for 5 turning 6, Year 2 for 6 turning 7, etc. In September 2013 when your DS will be entering the UK school system, Year 10 will be for 14 turning 15 between 1st September 2013 and 31st August 2014, and Year 11 will be for 15 turning 16 between those same dates. Therefore if turning 16 in mid September 2013, your DS will be put into Year 11 in any UK state school, regardless of the fact he has missed half of the GCSE course (year 10, or potentially 2 years of the course if the school starts GCSEs in year 9). This is non-negotiable.
My DFD started school in the UK, then did a few years in an American Middle School before coming back to the UK, but she moved back before secondary school so swapping curriculum wasn't really an issue. That said, she did find it very difficult to adjust at first; many aspects of the two systems are different. This is why I wonder as a secondary school teacher if you would be better going private/paying for an international school.
I teach GCSE pupils; the problem with a child going straight into year 11 without doing year 10, as your DS would be doing, is the fact that they have missed half of the course. It's not like in US High School where as I understand it from DFD all classes/grades are no longer than a year, with GCSEs, modules and controlled assessments completed in year 10 all make up part of the overall GCSE. So if your DS was to come straight into year 11 and just do the work that year, no matter how well he did in his year 11 exams and coursework he wouldn't be able to get good grades with 50% ish of the modules/controlled assessments uncompleted. There's also a lot of variation in terms of subject matter- even within UK schools, options from the exam boards in terms of what is studied in certain subjects, such as History and English, and how it is studied in sciences, mean that even swapping schools within the UK in year 11 would be a logistical nightmare.
It's a slightly different situation, but my DFD (year 11) came back into school a couple of months ago after missing the last month of year 10 and all of year 11 up until that point. The options she was given were to resit her GCSEs the following year in the school's 6th form alongside AS levels/btecs (only an option because it was so late in the year and because she had previously been a school dropout) or to work ridiculously hard between then and exams starting to catch up. She went for the work ridiculously hard and catch up to sit her exams this year option, which has meant rushing through controlled assessments after school, and sitting extra modules on top of the usual ones this summer, meaning she has extra to revise for including things she is learning for the first time during the revision process.
The point I'm trying to make is that although doable (perhaps, I'll reserve judgement until DFD gets her GCSE results!) catching up with missed GCSE work in order to meet the requirements to get the grades is an absolute nightmare for all involved. I've taught a fair few students who have moved to the UK in the past, but I've never had one arrive in year 11. Parents tend to avoid moving in that school year and go in Year 10/Year 12 instead just because otherwise their child arrives when everyone else has completed half of each of their GCSEs already and being held back a year in a state school (where I teach) is not allowed. As a secondary teacher, if I had your DS in my class next September I'd be very worried about him tbh.