I was wondering whether anyone - 22B? others? - had anything to say about how to manage the situation in which a child has done all the maths - or any other subject, if similar issues arise - that's normally taught at school, but isn't ready for university yet.

You could have him progress through theoretical physics and learn and practice the needed math along the way. For example in electricity and magnetism taught at the level of Jackson, one learns about solving partial differential equations and special functions. Quantum mechanics uses that math and also linear algebra, and there are applications of group theory. Computational physics requires numerical analysis and programming skills.

Some philistines

believe that applicability is what makes a branch of mathematics important or unimportant, so that following a math curriculum for physicists, engineers, economists etc. is a way of determining what to study.