@Seriously79 it depends if they find it hard. Ds1 sailed it, it just came very easily to him. For some people their brains just click with maths, both of mine did maths for fun whilst waiting for food in restaurants, mental maths challenges to each other. It was lovely to watch. Dh did a mech eng degree (maths and physics) and I did English Lit so this was all very alien to me too
I live in a maths nerd household but I love it.
Re Cambridge, look ahead now, I know it seems early but it isn't. He can do some stuff over this summer toward A level, just dip his toe, get a feel. Both of mine learned their Computer Science yr 13 gaming language for their coursework over their year 11 ten week summer so it was very easy for them later on. They just love learning.
If he is looking at STEP for a Cambridge course then I believe that Cambridge set the pass rate to deliberately drop excess students as they only have X number of places for their course. You can get the grades but still "fail" the STEP requirement. This might only be for maths, it has been a while since I had to look into this sort of stuff. Ds did CTMUA (Cambridge) TMUA for Computer Science.
@Landlubber2019 whilst it is extremely common to sit 3 A levels FM comes under the 4th A level category and when applying to university it is possible that a lot of the other applicants have FM if it is a maths based course and so it could potentially disadvantage your child.
Look ahead now at the university requirements and not only that I would look into how many applicants they offered to with FM or contact admissions and specifically ask. A university cannot specify FM because not all sixth forms offer it but a lot of applicants will have it. Ds went to Durham for Computer Science and something like 92% of all successful applicants had FM but it is not specified. He said you absolutely needed FM knowledge in first year. Some may have sat TMUA and provided their results too on their UCAS form.
Some unis ask you to sit an entrance exam, ie TMUA (The Test of Mathematics for University Admission)* *for say Econ at Warwick which is where Ds2 is. It does depend how high he will be aiming. Do not rely on staff at sixth forms being all knowing, do your own research and ask on here too as many of us have been through this as parents. I have learned so much from the secondary/further education and higher education boards. I was always looking a couple of years ahead to get informed.