Despite your request on a previous thread, I cant really help as both mine have taken/are taking quite vocational degrees, as are many of their friends.
That said one of their friends (Oxford humanities) is in a similar position and thinking of trying Civil Service Fast Stream. DS is one of those rare lucky children who knew what he wanted to do aged 12, though internships, University and peer knowledge have refined his ambitions. DD, in contrast, is slightly more ambivalent despite opting for medicine, and has a number of Plan Bs, including biomedical engineering or science teaching. (Actually her real vocation might be being a ski bum...) But also has a range of other interests, including cooking and dyslexia, which could well influence what she does, either in medicine or allied to it.
So...Civil Service (or similar like the FCA, National Trust, British Council, Local Government, NHS admin etc), teaching (Teach First is obvious, or perhaps similar experience in a boarding school, or even starting with a gap year with TEFL and picking up a second language), law, accountancy, management consultancy. Are there any sectors he is interested in: hospitality, tourism, property...
How about a really good career consultancy. DD did some computer tests organised by her school which provided an interesting profile of aptitudes and interests. It seemed pretty accurate, and what was also interesting was the big list of careers, depending on whether you were a people person, a practical person, good at detail, strategic and so on. Your DS could look at what was proposed and then decide which appeal.