I used to do admissions for Cambridge, for English. I don't see why anyone would be fussed he's done PE. English and History is a perfectly decent combination; I wouldn't care in the slightest. It would just come down to the interview, as with everyone else.
It is possible having done a different subject would help him be better prepared for interview, obviously - I can imagine, for example, that if he'd spent the time doing a language he'd have honed his linguistic skills, or if he'd done RE or philosophy he'd have honed more essay-writing skillls. But that's beside the point really; those skills can be acquired all sorts of ways, and what people are interested in is students who would be fun to teach, and who will respond will to the kind of teaching that's done at Oxbridge.
FWIW, the PP who says there's lots of Old and Middle English in Oxbridge English, and therefore Latin would be useful - well, no. There is lots of OE/ME in Oxford English. At Cambridge, no one studies OE unless they chose to do it (and it is not a popular choice); you also have to opt-in to study early Middle English (pre-1300), so what you are left with as a compulsory paper is later Middle English. Yes, some students find it linguistically complicated; yes, it is often easier for those who have done Latin. But it's one paper; it's not actually exponentially more difficult than Shakespeare, and I wouldn't advise a student do a subject they don't enjoy, on the slim chance it'd be useful for one paper at one university.
It is much more sensible for the OP's son to look carefully at what each English course at each university he's considering, entails. They are all very different, and nothing pisses off me admissions interviewers more than someone who rocks up with no idea of the course content and expresses their burning enthusiasm for studying something we don't, in fact, teach/ their utter disdain for something compulsory.