Hello! In terms of which A levels, chemistry and biology are definitely to be included if you want to keep your options open for all med schools. The third should be the one he thinks he can score highest in. My DD has done the bio, chem, maths trio because maths was the subject she felt she could score highest in. She also enjoys it and it's probably the one her teachers are most confident about her getting an A star in, so it was a good choice for her.
In terms of work experience our local hospital does a scheme for local students of a week in a team. They open the slots for the academic year around May/June time and they go really quickly, so it's worth checking the websites of hospital trusts to see if any do anyting like this and when their applications open so that you can get in as soon as they do. DD did this and was able to select the week she wanted (Feb half term in yr 12) and got her second preference of team (respiratory).
It's also worth seeing if any hospitals have any volunteering opportunities. DD tried for about a year to join our local hospital as a volunteer - she couldn't start until she was 16, but she eventually started in April this year. Although a lot of trusts will say volunteers need to be 18+ there are sometimes roles for 16-18 yr olds.
We found local GP practices less keen to have students because of the risk that they might know a patient, but if you were willing to travel you might find a practice that would agree to a few days of experience. My DD was lucky to get 3 days at a GP practice in Norfolk which takes medical students from UEA on placement in year 1, they had a 3 day programme of observing various parts of the busy practice. We were only lucky enough to get it because an old school friend of mine was a senior partner there - but I'm sure there are some GPs who would look kindly upon a nicely written letter. failing that the ObserveGP virtual wex that mumsneedwine gave a link to is meant to be highly regarded.
The main thing is what he learns from the experience. Understanding the role of a doctor and he challenges of the job and the skills and attributes needed is the important thing. The BSMS virtual wex (also link provided by mumsneedwine above) is good for reflection of what you have learnt from the wex you have done.
BSMS is great for webinars aimed at medicine applicants and if you sign up for their newsletters (you get a weekly one if you have started their virtual wex) then they often advertise other med school events as well as their own - DD managed to get on a Southampton uni hospital live virtual wex with them for two days in the October half term - they said they gave preference to widening participation students, which DD is not, but she sent off her application early and had to write a few paragraphs about why she wanted to do it, and was selected to do so. She really enjoyed it, got to see surgery happening and hear from lots of consultants about their jobs.
If your son is keen, he could try getting the BSMS wex under his belt over the summer as year 12 can be pretty full on, as you want to get the best predicted grades if possible, as that also helps widen the number of med schools you can apply to, as whilst some don't even look at them, some won't consider your application if you don't meet their minimum criteria.
I hope that helps and nice to have you on the thread.