Re 4 A levels, I would say you should only do it if FM is one because I think Maths/FM/Physics have so much overlap that they are not really the same workload as 3 separate subjects. If DS is considering Maths or Engineering some places require of have a strong preference for FM, but it is difficult, so probably easier to get a good grade in Comp Sci than FM. For medicine lots of places will not allow both Maths and FM to be included in the 3 A levels, I don't know whether there are other courses which do the same. I would say stick with 4 if he is happy to continue, but look at the summer exam results and UCAS predictions and make sure he knows that if he is struggling to get the grades for whatever he wants to do next, you are happy for him to drop FM. My DS has dropped FM as it has no value for medicine, but has started an EPQ, which he is aiming to complete next term, and picking up Y12 Chemistry in September. His school says that 3 A levels do not give you the required hours of learning, so they have to do something extra - EPQ, core maths, GCSE re-sits, volunteering all acceptable.
Re only applying for medicine with 10x9, that is nonsense in itself. If strategic, you can get a medicine offer with nothing over a 6, but you do need to be good at learning and have a good work ethic at university to survive the course, so if you worked really hard for GCSE and got all 5/6, it would probably not be a wise choice.