I'm not a tutor so if you posted in the tutor section hoping for one to reply then sorry.
Firstly the range of marks of successful pupils is likely to be tiny, with many sitting on one or two scores - the standardisation system makes this impossible to extrapolate but it is probable that many of the other successful pupils will have the same score as your child and the difference between 'pass' and 'fail' is really tiny - you are literally looking at tenths of a mark.
Secondly, there is no correlation between ranking on getting into a school and GCSE grades - several posters have put this on here in the past. Similarly, pupils who get into selective schools on appeal because they were borderline do not do worse than those who sailed in. Kids never discuss where they were in the rankings once they start school and it is all forgotten.
Thirdly, 'struggling' is a wide concept and most pupils who 'struggle' at school do so for a range of reasons usually only loosely related to their academic ability.
Stop fretting. If Pates is where you want your child to go, send him there.