And when the time comes, and if need be, to phone the universities and try to speak to admissions tutors
No, don't do that. It won't help, really. There are better times/people to telephone.
Universities receive A Level results a few days before the candidates. They use these to match offers to achievements of those who've firmly accepted their offer.
That's straightforward, and all those places will be confirmed.
Once they've matched achieved offers to places, they then look at the overall pattern of offers achieved in the university as a whole, then at each department/unit, with a view to whether they need to adjust quota across different subjects, what the capacity is for each subject, and so on.
With that information, then universities look at where there are opportunities for "relaxing" or "softening" grades. Because of changes in funding & quotas, if an applicant has ABB, and an offer, even if they haven't achieved the offer, they can be in a good position for their offer being confirmed. The government has removed caps on numbers of applicants with A Levels at ABB and above (before we used to have caps, and were severely fined if we over-recruited). Now, the limit on student numbers with ABB and above is the limit that the university can teach, rather than the government quota.
So, if a university has the places after they've confirmed the applicants who've made the offer, then they'll work down a list of those who just missed the offer.
If your DS is in that position, he'll be contacted. It's best to sit tight until you hear from your 1st choice/Firmly accepted place. Meanwhile, he has an Insurance offer. If he makes the grades for that offer, he has a place.
If he doesn't make the grades for either Firm or Insurance offer, and neither decides to relax/soften entry grades, then HE gets on the net, and the phone, and goes into Clearing. He'll have seen other universities and considered other courses in the course of choosing his Firm & Insurance -- so he'll have some idea of other places to study.
Or, he could go & work for a year, build up useful employment experience, and retake and reapply.
There are parachutes. A Levels are not the be all and end all. Neither is university.
BUT, there is no way an Admissions Tutor in a department, nor anyone in the central Admissions team at any university knows now what the situation will be in August. So leave it be. We're all flat out marking procrastinating third-marking failing dissertations and we just don't know how many places we'll have available to those who don't quite make their offers in August.
I think your DS is doing that human thing: projecting current anxiety about studying & exams on future possibilities. He needs to focus on the here and now.
(Former admissions tutor here: it's a complex couple of weeks, and Admissions tutors cannot take holidays in August at all -- makes for a long year ...)