@NoviceBuyer2022 - can I suggest you read through the medicine applicant support threads on here (starting from the beginning).
For example:
www.mumsnet.com/Talk/higher_education/4358647-Medicine-2022-entry
Although that is the second or third thread - so you might want to go back and read from the beginning. Similarly find the 2021 support thread. Both of these were threads for mothers who DC were applying for medicine. You will see a fair bit of me on the 2021 thread, as my DS, while on a gap year, as a deferred place for medicine at Nottingham University.
Obviously just about everybody on there had DC who were a lot older than your DS are - but it will give you a real sense. It might also become clear the extent to which the various medical schools change their criteria from year to year. Nottingham University ,where my DS has a place, has completely changed its entry criteria this year. So has Liverpool (another place my DS applied). Some of that might have to do with the A levels and GCSEs being TAGs, due to Covid, but some of it is that medical schools can and do change - and each one is a world unto itself. One will value GCSE grades, another the UCAT, another A levels - and you are only allowed to apply to four - so you need to pick strategically. Or at least that is how it has been for the last few years. Who knows, by the time your DS gets to Year 13, even that might have changed. But understanding the process of selection for medical school now might help you and your DS understand why people are saying that the key thing now is to concentrate on getting good GCSEs - when you understand that, at least in normal years, many, many medical schools use GCSE grades for a significant part of their scoring system to rank who gets offered an intervew.
On the 2022 thread you will come across the parent of an applicant who when he started A levels, thought he was going for a different career, and so didn't take chemistry, and how restrictive the medical school options were with that (I think there are only two medical schools he could apply to). Not taking biology also restricts the options. Certainly biology, chemistry and maths is a safe choice for A levels which keeps options open. There are something like 31 medical schools in the UK, and what my DS did in Year 12, was draw up a spreadsheet, and in that he put in all the entrance criteria for each of them (from their current websites) - and then also other things he either liked or wasn't so keen on. All that information is available on each medical school's webiste (although you often have to hunt to find out exactly how they rank for interview - not the minimum requirements, which are easy to find, but given that every medical school is desperately oversubscribed, you need to know how they rank). But the key thing to understand is that they really do change every year, so anything other than a general sense that GCSEs and exams taken in Year 12/13 are important will not necessarily help your DS.