OP, I am going to disagree with the post above. It is not about need, but about enjoyment and aptitude.
If you don't enjoy maths, or have sufficient aptitude you are well advised to stay away from FM, however much you "need" it. Maths is no fun at all if you are struggling.
But the reverse is true as well. If you have and aptitude and enjoy it, FM can be a lot more fun than GCSE. And, with the right aptitude a lot easier and less work than other A levels. And maths is everywhere. Data and computers have made this so. Even if, say, you are a linguist, but with strong maths, there will be a niche around linguistics, computer and language. The same is true for economics and will be for psychology. So it is not about which courses "need" FM, but which subjects you can use maths knowledge in.
This board seems fixated with careers in law and engineering. There are a lot of other avenues, especially for those with strong maths. And indeed the breadth within engineering is huge - just look at the range of courses available at Imperial.
In short it depends how good he is.
If he is good enough he should seriously consider FM as not that many people take it, or indeed are capable of taking it. Unfortunately that is hard to know before you start, though maths teachers often have a view about who has the aptitude as opposed to who works hard or is being tutored. (Tutoring, which is pretty rife across the board, does push kids to the top of the class who otherwise would not be there, which in turn can be demotivating for others.) Part of the problem is that GSCE is pretty dull and unchallenging for a good mathematician, so it is easy for kids to switch off. My DC both started to take off at A level, and I later realised that at GCSE they had never had problems with concepts, but had no motivation to do other than bob along, usually not in the top set, leaving keener kids to compete with each other.