@OrangeWoolCardiWrap Ds is at Durham studying computer science. His A levels were maths, fm, physics and computer science. He did computer science for GCSE so the A level is like an extension of that. It isn't like doing a new subject from scratch.
The way maths/fm is taught at his sixth form is they are timetabled 5 contact hours per week for each subject but it still leaves 5 hours for free periods, plus assembly, plus a tutor group thing. So it isn't like the timetable is rammed full. He didn't do an EPQ. They do the entirety of the maths A level in year 12 using those 10 hours per week. Then fm in year 13 sitting both exams in year 13 as the fm should also cement the maths.
If your son comes in on a grade 9 from GCSE it should be completely manageable says Ds. The main thing for A levels is keeping on top of the work and asking for help from teachers if he feels he hasn't understood something or is struggling with something. That is for all subjects. Durham ask A star AA but like the above poster the FOI will tell you Durham take very few students on that grade, usually higher so this is all about motivation and determination and a good work ethic. Ds says that it felt less than GCSEs workload wise and if you love maths then 50% of your timetable is a breeze.
Ds says fm absolutely comes into play for computer science at Durham. The vast majority of applicants to top universities will have further maths so this is who they are competing against. However, Newcastle and York have much less maths focus for their computer science, this is anecdotal from friends of Ds who are at those unis.
@Satsumalime sorry to hear about your son's health. A possibility is to continue with the 3 A level, ace those out, then take a gap year and do the further maths A level before applying to Oxford. Also in the meantime look at supercurriculars ie anything computer science related outside of his A level, there are summer schools some online, any TED talks, books etc. Does he have a specific area of interest computer science wise? He can add all of this and what he learned, what it led to reading/listening to etc for his personal statement.