This is way years ago but maybe will help.
Does he know what he wants to do for a degree assume going to uni?
For some subjects it's a massive help eg physics. So that's one consideration.
IF it's similar to my day, maths was more applied and FM more pure. I enjoyed FM way way more than maths. If it's similar now then they almost feel like different disciplines, the problems you work on and the principles you use are different. So while both maths, what you're learning to do and what for is not really the same.
I know things are really tough these days. In my life though every time I've gone for a job interview the fact that I have a FM A level. Even though got a poor grade. Is generally taken as a sign that you are pretty clever. It's also usually brought up and that gives an easy thing to chat to interviewers about. Always good as then you have an opportunity to show how engaging and interesting you are! Ditto physics degree.
However most important to last.
He likes it. I imagine the school would tell you pretty fast if they thought he wasn't up to it? Given the huge focus on their stats these days.
Let him do his thing. It makes him happy. Enjoyment is so important in education. I'm a bit of a do what you love person. Rather than things you are less interested in because you might get a bit of a higher grade.
Further maths was when maths really came into its own for me. I was always good at sums and acceleration and angles and etc all that stuff.
But FM was when it got interesting.
Anyway that might well all be out of date but it's from the horses mouth iyswim!
Good luck to him :)