Well I think I've answered my own question.
"I think prolonged and varied work experience is useful for potential medics because it gives them a better idea about whether it's the career for them or not. A week here or there may be ok for the application but it's not really enough to get a proper idea of what the job is really like". (Wonky)
"Medical schools will not want to use the same selection criteria, as then they would all be picking the same students. They will also want to select those students that are right for their courses.
The NHS will be concerned that those selected know what they are letting themselves in for, and will want to remain within the NHS long-term.
Given Stranger's DS is clearly very bright, my guess would be that the one offer he did not get, was from a medical school that was looking for a different ratio of practical experience to academic". (Needmoresleep)
"However volunteering etc will not usually prepare you for the other aspect of clinical practice. Doctors have to cope with being able to rapidly process large amounts of information and demands coming from all directions and make often instantaneous decisions in very stressful, tiring situations. Those decisions cannot be wrong most of the time because of the implications of what happens when they are. It takes a certain type of person not only to survive that but to maintain calmness and continuing career satisfaction despite the pressures. It obviously helps if you are somebody who thrives under pressure and are a rapid processor (academic as well as practically) with a good memory". (Oxfordmedic)
"DD says that judging from the shrinking size of tutor groups, there were a number of drop outs during the first year of her course". (Needmoresleep, re. Bristol).
All of the above are extracted from the thread, offering alternative takes, but dipping in and out of various pieces of info freely available on the internet - and some retention rates seeming to be completely appalling - it's pretty clear that the ratio of academic to practical has a direct bearing on the retention rates, even though this info gathering is done at a very amateur level. A slight concern given the opening up of medical school places and the debate about falling standards....
So OP, I think perhaps the thing to focus on is your DS's aptitude for medicine, not how often he needs to show up at a care home or how many days he needs to shadow at a hospital. Aptitude goes to essential qualities and not to attributes.