To get onto a medical degree he would need:
- Top GCSE grades in all his core academic subjects, including maths and science.
- Top A level grades in three A level subjects including Chemistry and ideally at least one out of Biology, Physics or Maths.
- Good relevant work experience.
Even then he'd be lucky to get an offer the first year he applied. It's hard to get into medical school and it's not a walk in the park once you're there. The dropout rates are quite high and there are years of further training and exams to get through after completing the medical degree. Having said all that, once you qualify you can usually be pretty certain of having a job for life and the salary is very good by most people's standards.
For a Psychology degree, the entry criteria are a lot lower but it does help a lot to have a good grounding in maths and biology.
He should have a good look at the website of the British Psychological Society which will explain what careers are open to people with a degree in psychology. Clinical psychology is not the only one overlapping with psychiatry - counselling, forensic or health psychology might be areas to consider. It all depends on what appealed to him about being a psychiatrist.
Other specialist areas in psychology are educational psychology, sports psychology and occupational psychology.
The big difference between psychiatrists and psychologists is that psychiatrists prescribe drugs and psychologists do a lot more work in the area of psychometric testing (e.g. assessing different types of intelligence) and therapy.
Other areas he could look at that would get him into working with people with a mental illness are mental health nursing, occupational therapy and mental health social work. None of those are (to my knowledge) as tough to get onto as medicine or clinical psychology. A bright, capable mental health nurse would have an excellent chance of moving rapidly up the ranks and would probably end up earning about the same as a clinical psychologist. It is not a job for the faint-hearted, though.
Clinical psychology is incredibly tough to get into. It is common for each training scheme to have hundreds of applicants for 20-40 places and for those who get accepted to have not just a good first degree in psychology (a First or a very good 2.1 would be expected) and some relevant work experience but also a good Master's degree and (often) a Ph.D. as well. It's extremely common for psychology graduates to apply for years and to be well into their 30s before getting onto clinical training (or giving up and doing something else).
The reason it's so competitive is that it's funded by the NHS and trainees get a salary of about £30k pa as well as all their university fees paid while they train (which takes three years). It used to be the case that on qualifying clinical psychologists should be able to get a good job straight away, but psychology has been very hard hit by cuts in NHS budgets and this is not necessarily the case any more.
I think your son would be well-advised to try to get some work experience to help him sort out what he really wants to do. Good luck!