Is there such thing as dual enrollment in the UK?
After finishing the usual highschool sequence, my son took the undergraduate math courses at the university, but as a highschool student. When he matriculated and started as a freshman at his current university he started taking graduate level courses. He has never had to repeat anything. He has also self studied topics and has been allowed to skip taking the courses formally by showing mastery through discussions with the professors and, in two instances, taking the course final exams. ( didn't receive credit, just was able to waive them as prerequisites to higher level courses)
Would any of that be possible in the UK?
Short version: nope.
Long version: there's the Open University, but its courses are not really suitable for a very mathy person. Anything else would be a special one-off arrangement. I know of one person, decades ago, who did have an arrangement that led to him getting a maths degree while still at school, but he had a parent available to drive him back and forth, and it was less flexible in various ways than what your DS had, I think. ETA a key element is that university here doesn't work by accumulating credits. There are courses you take in each year, and then you're done. Doing something different requires special permission and may or may not work practically. "Graduate school" is completely different from in the US; a PhD student here doesn't typically take taught courses at all, so graduate level courses don't exist in the same way, though much of the material to be found in US graduate courses is in undergraduate courses here and there are master's courses.
That's interesting, thanks for that pointer. (But ouch, on top of school fees!)
Edit to answer your edit: yes, there is plenty to be said for free learning. It may be that this will solve itself that way. Against it is the relatively inflexible university system he'd be going into if he read maths here later, as above. One way this could go wrong is if he'd taught himself half a maths degree, and then had to sit through all that again to get the other half. He says now that he'll probably want to read a science, in which case the issue won't arise, but he looks like such a mathematician in how he thinks that I won't be at all surprised if he changes his mind.
