I agree that Maths and Philosophy Joint Hons is definitely worth exploring.
I went a different direction, into a very maths heavy STEM subject. In my home country, we did much more maths for my subject than British students do. When I eventually came to the UK as an academic, I had my own research line where a different way of looking at things has worked out for me.
When you write papers, putting some effort into writing well and getting feedback for this can be satisfying. A number of scientists and mathematicians do public outreach of various forms. Writing skill is crucial here, and SM forums offer many opportunities to grow as a communicator.
But that in the future. For now, I think your DS needs to pick a tentative primary focus. As a scientist of course I prefer that he pick maths, but both choices are equally valid.
In favour of math I will say that it is easier, pragmatically, for a mathematician or scientist to get seriously involved in writing and the arts than for a humanist to gain any realistic appreciation of mathematics or an intensely mathematical field.
If DS wants to maintain a foot in both camps, I think Maths and Philosophy or a degree in Maths or a Maths intensive science, with self directed involvement in the Humanities, is his best bet.
If he wants synthesis then obviously PPE, Economics, Politics, Sociology and possibly some aspects of medical and education research could be what he is looking for.
He sounds an interesting lad. Best wishes to him.