"I think that Oxford can be an extremely tough environment for people from comprehensive schools. Not because they are not clever enough but because the social class difference is really obvious and it's impossible to automatically fit in - even if you want to."
Sorry but I really have to disagree with this. I note that the person who posted it didn't even go to Oxford (and admits to having a chip on shoulder about that). There are plenty of people from comprehensives at Oxford - more in fact than at many other universities (eg St Andrews, Exeter, Bristol which are filled with public schoolers). And even if there weren't - it is possible to get on with people who went to a different type of school you know! I went to Oxford, and have a very close group of friends from there which includes comprehensive schoolers, grammar schoolers, private day schoolers, public schoolers... you name it.
Anyway. Back to the OP.
Oxford really is NOT the be all and end all. It has plenty of downsides. Lots of independent work and comparatively little teaching; heavy workload so less time for extra curricular activities; degree outcome almost entirely dependent on Finals (eek), little or no coursework; not the most modern approach to many subjects; quite a lot of weirdos as students (!); and so on.
Oh, and I really don't think Oxbridge vs Russell Group makes much if any difference from a job point of view.
Applications to Oxford are, in many ways, a lottery. There are more qualified applicants than places. They have to choose somehow. It is often just sheer luck. Your son was unlucky.