I assume that by 'final year' you mean Year 11 (16 years old) rather than Year 13/Upper 6th form (18 years old). If your son wishes to train to be a doctor, I hope that he has been studying appropriate subjects at GCSE level. In order to move forward to take appropriate A-levels (either in the school 6th form or at F.E. college)and to then be able to later impress a university selection panel, he'll probably need the following GCSE results:
Maths (As a former Maths teacher, I'd be very worried about anyone going on to study A-level Maths, or any science subjects, with anything other than an A grade at GCSE).
English Language (A or B grade)
Chemistry (A, or possibly B, grade)
Biology (A, or possibly B, grade)
Plus at least 2 (preferably 4 or 5) other GCSEs (possibly including Physics) with A or B grades.
What really matters, however, are A-level results. These are as follows:
A-level Maths is essential. A further two A-levels in science subjects are also part of the normal minimum requirement. (Chemistry is often regarded as a particularly relevant subject, even ahead of Human Biology). The (theoretical) minimum grades for these A-levels are an A and two Bs. In practice, many (probably most) students who get onto university courses in medicine have 4 good A-level passes (often all with A grades).
It's possible to get onto a medical course without such a scientific background at A-level (although most universities would probably still require A-level Maths and there would still be a minimum requirement for only A or B grades). In this case, however, the university course would last for 6 years, rather than the normal 5 year period.
Further information is available here. (click the 'GP', 'Hospital' or 'Surgeon' link, as appropriate):
http://www.learndirect-