OP, my son sounds exactly like yours, in being a good all rounder, and he likewise got mostly 8s and 9s at GCSE. He was unsure of careers beyond knowing he wanted to work in finance or engineering (he definitely didn't want medical or humanities), so he ended up chosing A level Maths, Physics and Economics which he thought gave him the broadest choice.
It was during Uni open days where he explored Physics/Engineering related degrees and decided they weren't for him, so that left Maths or Finance. He toyed with the "Maths with Physics", "Maths with Economics", etc. He was predicted A*s at A level, so was looking at Uni courses requiring straight A's, which discounted most business, economics, accounting, where the entry requirements were usually a step lower, and he wanted the "best" and broadest he could get. He didn't want to do a "Maths" degree on it's own as he isn't really that excited/interest in Maths especially the heavily theoretical stuff - he is a very practical person and not good with abstract concepts!
He finally came upon "Financial Maths" which is fundamentally a Maths degree, a Bsc, weighted towards financial aspects, and included modules for economics, accounting, computing, etc., but more than 50% were "maths" modules, mostly statistics, probability, etc. He barely had to do any work for the two economics modules as he'd done the core content in his A Level, so just basically "half watching the lectures", a bit of revision and doing the course work, essays, etc rather than actually learning the two modules. Likewise he found the accounting/business modules pretty easy and not too time consuming. It gave him more time to concentrate on what he called "hard maths" modules which, to be honest, he did struggle with.
It's worked out well for him, he's just heard he got a first, and he starts on a really good actuarial graduate scheme with one of the UK's biggest insurance firms this Summer. So, A level and degree choices turned out spot on for him.
OP, please suggest the "Financial Maths" degree to your DS, it sounds right up his street. Also, economics will be far more useful than Physics, and makes Uni life easier in not having to spend too much time studying for their economics modules (that will be the same for other degrees such as accounting and finance etc).