The reason the same degree at different universities may be valued differently is that not all universities are the same. In various ways, depending on your criteria for measurement, some universities are better than others.
Other posters have mentioned all sorts of things which your son might consider as his criteria for choosing particular degree courses. It's important to think about them: the kind of environment he'll be studying in, city location, campus or green field site, small or large, sporting facilities, orchestra. All those things are important to individual students. But I'll now talk about the thing that a lot of people prefer to gloss over ...
But the main thing is that a lot of universities were polytechnics before 1992. They taught a range of largely vocationally oriented qualifications, not always Honours degrees. Some of them are extraordinarily good at what they taught, and still are. However, their staff were not funded to do original research. So they taught from others' ideas, rather than their own research. They are now funded (sort of) to do research, but the results of the Research Excellence Framework (REF) show that, generally (there are exceptions) there are "pockets of excellence" in the post-92 universities (sometimes called "new universities), whereas the older, established universities have a much greater depth & breadth of research achievement.
Research is new knowledge: stuff that no-one really knew or recognised before. It's one of the principal functions of a proper university, and feeds into teaching in a number of ways. It means undergraduates are taught by staff and Doctoral students who are engaged in developing ideas at the edge of the known field in their discipline and this excellence filters into teaching. Imagine being taught by the PhD student who made the recent discovery of a possible eye-witness account of Mohammed in a manuscripts collection at the University of Birmingham? Imagine being a first, second or 3rd year undergraduate and being part of a community where that sort of discovery s being made.
So if your DS is thinking of a generalist subject, then he should aim for the universities which are hardest to get into, and which score highly in a range of league tables. Don't rely on just one. And then look at the staff who will be teaching him. YOu can usually look up the staff profiles in a Department: see what books they've published and when (how recently?), what research grants they've won from the various Research Councils, and whether there are other sorts of awards and prizes eg from the Leverhulme TRust, or the Wellcome Trust, or the European Research Council. And so on.
If your DS is currently interested in "Modern Studies," then a good History or Politics degree would give him that sort of focus, but also huge scope for developing interests.
But if you're aiming for a generalist arts/science subject, it's best to go for the best you can manage: and that means highly rated staff research. Because that means that the general standard of teaching & learning will be higher than a department were original research is not the norm.
What I've said here will not please some MNers who resolutely hold to the idea that top flight researchers are rubbish teachers and research gets in the way of teaching. That is not only an incorrect assumption about good teaching and good research, but it's also an anti-intellectual, anti-educational one. It shows a lack of understanding of what a university education is about, and how it happens. We are training undergraduates to become graduates in a knowledge economy. Even if they don't go on to be researchers in university sense f the word, if they enter a profession, they'll be working with knowledge, ideas, concepts. THey need to be trained, challenged, pushed and encouraged by people who do this all the time, and in their own work. And as a stereotype of the reverse of "brilliant researcher/rubbish teacher" I offer you London Metropolitan University: "rubbish research/rubbish teaching.' And very low student satisfaction scores by whatever measure you want to use. And a stererotype just as inaccurate as the "brilliant researcher/rubbish teacher" one.