His schools does not do SATS so we have no official proof of ability apart from reports from his teacher & tutor. Is there any test which I can get done for him to use as evidence for his English skills?
I can't think of a curriculum type test, although an educational psychologist could test for verbal reasoning skills. Reports from school might be sufficient, together with his English exercise books. I wouldn't use a report from a private tutor as it's unlikely to be viewed as independent.
Can we ask for a copy of the exam paper as part of our appeal process?
There is no legal entitlement, and most admission authorities would certainly refuse.
Can we ask for a breakdown of the English marks to see what went wrong?
This might be possible. Worth asking.