Hubble, I would hope the interviewer had the presence of mind to either choose one of the questions on the reverse side to get your DS to talk through in a tutorial style, or junked it all and went on a different line of questioning.
He's applying for PPE, is that right? Did that mean 3 separate interviews? I had to do couple of philosophy interviews for my course when I was applying - they were terrific fun: hypothetical questions, no right answer, just chew the fat with the interviewer(s). Hopefully your DS felt the same - they're called interviews but (in my admittedly rather focussed experience) they're really more like conversations.
itinerant, on the 'why oxford?' question, the intent of that (as I'm sure you appreciate) is to offer up a soft serve to ease you in to the interview, before the meaty bit: it's definitely not intended to be some sort of litmus test of 'is this person saying the right thing, do they want it enough?' I'm sure whatever your DS said was sensible enough - and will not be the difference between acceptance and failure.
On a broader note, as I'm sure has been hashed through already in this thread - the tutors are not looking for answer to their questions that are going to blow their minds. They're looking for inquisitive, curious minds, and teachability - basically, I've got to teach this person, potentially for 3 or 4 years. Are the both of us going to get something from the experience? If you make a mistake in your answer, were your thought processes rational? If your DC have engaged with their interviewers, then they can feel they've done themselves justice, however they felt it went with respect to answering questions 'correctly'.
Good luck for tomorrow all - Lobster, I'll be thinking of your classicist, hope it goes well!