Hello, OP -
I am a former Russell Group STEM admissions tutor. I think the advice above and your own diagnosis of oversaturation are likely good, and accurate.
DS need not put all of his choices down at once, so he put down Cambridge, Warwick and Durham now and think further about the other two choices. I do believe strongly that he needs an offer no higher than A star A A, and preferably AAA, that he feels enthusiastic about.
One thing he could do is join the online undergraduate forum The Student Room. There he can pose questions directly to current UGs. The discussions are generally thoughtful.
Some SoMs that seem to me like good Insurance choices for very strong applicants include Bath, Lancaster, Edinburgh and Glasgow. Glasgow Maths did extremely well in the last Research Assessment Framework, coming tops in Scotland and well within the top 10 in the UK. Lancaster was also a UK top 10. These are both breakthroughs, but both SoMs have very good histories with Teaching and Learning and student satisfaction. High satisfaction plus research vitality is a potent combination for strong students.
Sheffield and Manchester are also possibilities. Sheffield has excellent student satisfaction and plenty of strong individual researchers; overall, its student body and staff research impact are not quite at the same level. But honestly, for undergraduates that matters much, much less than the ability to get a great education and Sheffield clearly offers this. Manchester overall has stronger students and staff, but lower student satisfaction.
Thinking about whether he wants a campus or a city university may also help to clarify DS’ choices.