Do check out UCL's Arts and Sciences Degree. It's loosely based on the US degree model where students can mix arts ands sciences.
^
www.ucl.ac.uk/prospective-students/undergraduate/degrees/arts-sciences-basc^
From the UCL website:
You will choose a major and a minor pathway on application. Your pathways determine the subjects in which you will specialise. We offer two arts pathways and two sciences pathways.
Cultures (Humanities and Arts): Anthropology, Creative Writing, Cultural Studies, Design and Visualisation, Drawing Methodology, Film and Documentary Film-Making, History, History of Art, Languages, Literature, Philosophy
Societies (Social Sciences): Archaeology, Economics, Geography, History, International Relations, Philosophy, Political Economy, Politics, Sociology
Health and Environment (Sciences): Anthropology, Biochemistry, Biological Sciences, Biomedicine, Environmental Sciences, Geography, Geology, Neuroscience, Pharmacology, Psychology, Science and Technology Studies, Synthetic Biology
Sciences and Engineering (Sciences): Chemistry, Computer Science, Earth Sciences, Economics, Engineering, Mathematics, Physics, Science and Technology Studies, Statistics
^
From the above he could major in say Comp Sci and minor on Film Making or major in Physics and minor in International Relations. It's a competitive course to get into but it looks like something that would suit your DS.
Another option might be the many Physics + Philosophy or Maths + Philosophy degrees that are available.
The WhichUni website suggested Economics as the top option when I put in your son's A-Levels
university.which.co.uk/a-level-explorer/history/mathematics/physics?o=201
Other options included a Philosophy degree and Medicine (which I'm almost sure he couldn't apply for with those A Levels)
Anyway, hope the above helps!^