What HostessTrolley said.
The most important things are getting three good A Levels ( or realistic predictions ) and a good score in the BMAT/UKCAT. This will get him an interview.
The work experience and shadowing will help him do well at the interview and get an offer. Being able to balance all these things is important.
Remember that these things come in this order, for most student applying straight out of school:
Do first year of A level studies AND work shadowing AND paid work AND contribute to life of school, hobbies, sports etc
Summer - research universities, go on visits, prepare for entrance tests, Sit UKCAT and BMAT, get results, write PS.
September - put UCAS form into school .
October - March .Wait and prepare for and do interviews while doing 2nd year of A levels and all of the above - paid and voluntary work, sports, music etc.
August - get results and place
September - leave for uni
This is a two year process minimum and the A level grades part is right at the end. Nothing seems to happen for a year and then it all happens in a few weeks.
The grades are important but they are not everything by a long way. There’s no benefit to doing four and quite a few disadvantages.
Even if you are not sure of one subject, it’s better either to swap for a safer bet or, if its a compulsory subject, do some extra work / get a tutor / more help from school.
Remember many medical schools will only interview those who score in the top half of the entrance test. That might sound easy but remember everyone sitting it is a prospective medic / dentist - they are all smart. Either they have the grades already or they have a good chance. Quite a few are already graduates. It’s a tough field.