I studied Ancient & Modern History at Oxford, and would say that generally the joint courses eg History and something else are a tiny bit easier to get in on, just because there are only 1 or 2 students/year per college and so they are viewed a little bit more as individuals - make sure you pick a college with a tradition of taking students in that joint subject, though!
I don't recall the interview being particularly 'tricky' - being able to speak with confidence beyond the limitations of the syllabus is useful. But there is no need to be overawed.
Your son needs to sit the entrance exam,I believe - presumably his school will tutor him. You can help by talking about bigger questions, getting him to read newspapers etc.
Second (third? fourth?) the advice to visit colleges and find ones where he likes the tutors and where their research interests ideally match his own interests - at the end of the day, decisions are made on the basis of personal likes and dislikes - tutors choose candidates they feel they would like to spend time with for the next 3 years, so one where he bonds with the tutor on an individual level, or at least where s/he seems pleasant and encouraging (and not all are!) is a good start.
Be aware, if your ds is interested in 'older' history, that the Oxford course artificially starts in 285AD, or did when I was there, which always seemed odd to me,and was why I chose the joint course I did - Cambridge's course has no such limitations, as far as I am aware.
Be aware that Oxbridge history is simply far more work and far less guided than other unis - I found it hard emotionally as well as intellectually, and arguably would have had a better time elsewhere - not sure I would recommend it unequivocally.
I would definitely NOT recommend Durham - it's notorious for being the home of Oxford rejects,as the first anecdotes on this thread suggest - if your ds fancies endless conversations along the lines of 'so what college did you fail to get into, then?', then he'd be right at home. MY DH went to Durham and hated it as it was indeed stuffed to the gills with public school Oxbridge rejects (he was a state school non-Oxbridge candidate).
Good luck to your ds.