It's interesting to see the sort of information which is given out about Oxbridge admissions by schools and parents. It doesn't always match my experience as a tutor at Oxford in English Literature.
Empathetic is exactly right: "The type of school your son attended and the subject he is aiming to study will both affect Oxford's expectations re GCSES results." We are given some contextual statistical data about the school which allows GCSE results to be put into context. We are also conscious that there are some late developers who will not have a perfect profile at GCSE but are on a trajectory towards greater things. The grammar school information about 6 or 8 A*s may be right for that school, but not for all schools and circumstances.
For my subject, the decision as regards who to interview comes down to a combination of factors: GCSE results, AS grades, predicted A levels, school ref, personal statement, written work sent in (this is important) and aptitude test (also important). Each Faculty website should have some information about the criteria they use for each subject. You can see that for English GCSE results would only be one part of the decision.
I realise it is asking a lot for applicants to put down Oxbridge as one of their places if they don't stand a realistic chance, but my experience is that often assumptions are made about our requirements which are not accurate. We are looking not only for achievement but for potential later down the line. GCSEs are a predictor of later achievement in many cases, but not exclusively, and we are really looking to see how people do at interview (which offers a brief 'model' of a tutorial).
I hope this helps - careful research on each subject website, and asking tutors at Open Days, will I hope provide the most accurate info.