CS lecturer in RG.
A few universities want further maths. Currently Imperial and a few of the Cambridge colleges, but I suspect Warwick will join them shortly. Anecdotally, it's a line they find hard to hold and they offer a lot of alternative routes (STEP, in particular) but I suspect they will get harder about it over the coming few years.
The new Computing A Level is very good, and recommended by many places (including the above places that want FM). I suspect that the very best students have M, FM and CS, but I don't know: we don't ask for FM and don't get many students with it.
A few places will currently accept Computing or A Level maths, plus two others. Without wishing to get involved in crystal-ball gazing, I think that situation is unlikely to persist and the Russell Group and the next tier will have A Level Maths as a hard requirement, as it was in the past, within the timescale your son is concerned with. Courses you can access without A Level maths are making other compromises, and in any event students who do not have A Level maths fail the first year at a wildly disproportionate rate (note: they aren't comparable in other ways, tending to be first-in-family, so this isn't as clear-cut as the numbers imply).
If my children wanted to follow me but keep their options a little bit open I'd recommend Maths, Physics and Computing (which is what I did thirty-five years ago) and add Further Maths if the school permitted it. If restricted to three A Levels, then I'd also consider Maths, Further Maths and Physics and do Computing as an external entrant or just keep up a hobby interest. That way you have all of computing, all the engineerings, maths and physics open to you, at all universities (there are no institutions demanding A Level computing, mostly because it's not remotely universally available).
But in summary, if you don't do A Level maths, as of today you are excluding almost all the RG computer science departments and a lot of the next tier, and in two or three years time that situation will be even more clear cut (places that on paper say they will consider candidates without A Level maths in practice either don't', or impose additional conditions you may not find congenial).
Oh, and don't let anyone try to convince you that BTEC IT qualifications will get you on to selective CS degrees. They probably won't, and if by some miracle you manage it, about 75% of such students fail the first year. There are claims that starting next year BTEC IT is being "improved". We'll believe that when we see it, and are not planning to change our blanket refusal to accept it.