I wouldn't think about it in those terms, really. If he is serious (and I speak as someone who changed career plans about 5 times during high school) then he needs a school which will get him his results at GCSE and A level. He'll also need suitable work experience later on (some schools have better links than others - I spent 2 weeks in the operating theatres in my local hospital, I know people from other schools who were sent to the local supermarket/factories to sit it out!)
Then there's extracurricular activities - go for a school with plenty of choice, it doesn't matter so much what they are as long as he does something, so he should play to his strengths be they music, drama, sports, scouts, DofE, cadets, whatever (a range is good too - dabbled with all the above myself) They like voluntary work too, once he's older shifts in a nursing home etc always go down well, and are increasingly expected! WRVS may take him earlier (it's mainly but not exclusively little old ladies who help running shops, cafes and libraries in hospitals, changing flower-water and generally being GoodEggs)
The only thing I'd say about academics is to try for separate sciences at GCSE, as it makes A Level easier. Look at prospectuses before choosing A-level options, as some have different requirements. All want chemistry, but some maths and some biology, so check out his favoured ones at that stage.
On a practical note, medicine is a long course as an undergraduate, and longer learning after that; he needs to understand that the Daily Wail headline salaries are effectively made up (so he'll be paying back loans for years - I missed the start of fees but due to finish paying next year after 8!), that there's a lot of moving around the country especially to start with after graduation, and exams continue way into his thirties. And I still work 100 hour weeks on occasion, even if not routinely any more!
If he's still up for it - GREAT! I can't imagine doing anything else, and I look forward to working with him in about 15 years time!