Hi, OP -
Former Russell Group STEM admissions tutor here. Maths and Physics opens so many options I won’t add too many to the lists above.
You haven’t said how DS would be feeling about Law were it not for his poor Mock result. I mention that only because I thought @AllInADay ’s suggestions of Law specialisms were very interesting and somewhat unusual. If DS has low PGs and wants to read Law he will probably need a gap year, but that is not in the least unusual and it can be highly beneficial.
As @clary suggests, the reality many places, but perhaps not post 1992 unis, is that most Maths students will have done FM. This statistic might be publicly available. Given his current crisis of confidence, your DS might have a hard time in a situation where (he perceived that) he was underprepared compared to his peers. However some post 1992 unis offer an excellent Maths education. The FM dichotomy is true to a lesser extent in Physics as well. Again some post 1992s are very good.
Two further possibilities; I will second @ShandaLear ’s mention of various Computing related degrees, from CS itself to quite niche degrees at post 1992s. Maths and Physics are the most important ALs for CS and CS AL is not required anywhere that I know of. The second possibility is Mechanical Engineering. This is one of the broadest Engineering degrees for employment purposes and amongst the most mathematical.
It sounds like you are also not far from Liverpool, Hope, John Moores, Lancaster, Sheffield, Sheffield Hallam and Leeds. Sheffield is notable across the Engineering disciplines.
However if DS did originally lean towards Law and now feels put off, I wonder whether he would a Business degree with a small Maths component? That little bit of STEM is real value added for employers if well chosen.
The main thing is that your DS needs to get his confidence back. Starting uni in a positive frame of mind is important, even if takes an extra year. Good luck to him