Looks like another Bucks appeal.
I'd be less than honest if I didn't tell you that the absence of mitigating circumstances in your case will make things very difficult. But I think you've worked that out already.
How many below your son qualified? And were they immediately below?
The head ought to put the year 5 SATs on his summary, but if not you can show the panel some evidence (were they given out with the year 5 report?).
Is your son exceptionally good at Maths, and is there any other strong evidence for it? I'm just wondering if you could try and make a case that his sort of ability does not show up in an 11+ based solely on VR. I recall a case where a child had taken part in a Maths olympiad, and the panel accepted this argument!