I empathize with you entirely OP, and I guess this must be a very stressful time for your DS and yourself. Essentially, all that?s got to be said have been said here on this thread and I have nothing much else to add except to advise to ?look outside the box? so to speak, in addition to all the good advice already given.
(1) All UK medical schools work closely with the General Medical Council (GMC), the controlling body governing doctors in this country and it would be useful to get into their mindset and see what they, the GMC want in a new doctor.
Quote: The GMC sets the knowledge, skills and behaviours that medical students learn at UK medical schools: these are the outcomes that new UK graduates must be able to demonstrate. The GMC also sets standards for teaching, learning and assessment. These outcomes and standards are laid down in Tomorrow's Doctors (2009) (pdf). The guidance is also available online. [GMC]
The GMC have published a comprehensive guideline entitled, ?Tomorrow?s Doctors? giving a clear insight into the calibre of future doctors that would like to register.
Without doubt, understanding the full report and gearing the UCAS application towards the GMC?s ideal would surely impress the admissions tutors no end. 
(2) Don?t know how feasible it is for your DS to attend but in a couple of weeks time there is a ?Medical Careers Day?, a one day conference organised by the Royal Society of Medicine in association with Eton College entitled, ?So you want to be a doctor?? Many big guns from the medical world/schools such as Imperial College, Oxford University, Barts and The London School of Medicine and Dentistry, etc. will be there guiding prospective medical students on how to make a successful application to medical school. You can view the programme here.
I?ll be taking a promising kid from a state school there which is what this conference is aiming at. But whatever, I wish your DS every success this time round. Good luck!