I have just been looking at clearing for maths, just in case DS doesn't make his offers.
I was interested to see that the universities were listing UCAS points rather than grades as their requirements. I think that the discussions on here last year were about grades rather than points, that is an interesting change if so. There didn't seem to be mention of maths or further maths required to a specific level.
It looks as though there are some quite decent universities offering maths courses, but not Imperial/ Cambridge/Oxford? Warwick.
For example, Manchester, Southampton, Queen Mary London, points 144 or lower.
OP, if your son has a place a a Russell Group University for a competitive subject, it might well be asking whether they would allow him to switch. Of course, they may say no, but there is no harm in asking.
A schoolfriend was allowed to change from Law to English at Oxford (many years ago) for example.
If the university in question asks for maths entrance papers etc, then it may be more complicated, but it is worth exploring.