Medical schools are inundated with applications but won't want to all select the same people. So they filter slightly differently. Some (Keele, Bristol) like lots of volunteering or extra-curricular, others (Oxbridge, Imperial, QMC) seem to want tip top academics, others rely heavily on aptitude tests.
Very few applicants meet everyone's criteria. The trick is to apply to the right places. You can only apply to four, and you only need one place.
I would look closely at the entrance criteria for each, perhaps looking at this summary natalierm2707futuredoctorblogs.files.wordpress.com/2016/08/a100-2017-entry-admissions-policies-nrm2707-fdb1.docx and then looking at the detail on a medical school's website.
(I would also recommend the blog natalierm2707futuredoctorblogs.wordpress.com/category/applying/ written by the same medical student.)
Also look at the applicant to offer ratios on The Student Room. They can vary widely with Belfast and Bristol on opposite ends of the spectrum. Unless you are very confident try to limit yourself to one aspirational choice. (And don't apply anywhere where you won't meet the criteria.)
There are several Universities that don't require biology. Cambridge is one. Lacking chemistry is more of a problem but it looks as if Cardiff, Keele and Newcastle don't insist. The trouble with missing one of these subjects is that your choice is then limited and you need to ace their other criteria.
Provided your DS can find four Universities that he wants to apply to, which don't require biology, I would not worry. Get on with the maths and work hard at getting good predictions and ticking as many other boxes as possible.
Maths is increasingly important, and also gives him a good range of alternatives degrees. A growing and interesting field is bio medical engineering. DD was told much the same as CMOT suggests. Indeed the researcher (of artificial hearts) who spoke to us suggested that all medics should think carefully about whether they are interested in the science or people. Medics are like car mechanics, they keep the body going, whereas people like him are more like the car designer. Compatible advice from a medic friend was that a medical degree is not a bad place to start. So if DD retains her interest in maths/physics/electronics and finds herself less interested in general medicine she could intercalate, and use this to steer her career. The more maths you have, the wider the options.
Volunteering is considered another potential barrier to those who are not certain about medicine. My personal view is that not being sure is a better place to be, than deciding a decade before that you want to be a doctor, working hard to tick the boxes, thrilling your friends and family when you get a place, and then hating the course. Honestly do as much as you can, but don't worry too much. I would look around now, and perhaps pick a couple of things (ask your local leisure centre if any disabled sports groups use it, and volunteer to help out at a weekly session, or perhaps ask about a local befriending service and visit an elderly person once a week or do their shopping) which you can do consistently and which wont take up much time. DD volunteered one holiday in a care home, cooking and cleaning and occasionally taking someone with dementia out for walks. It never came up at interview but gave her an important insight into whether she could cope with the sort of patient who makes up a significant portion of a hospital population. (She could but lots can't so you might as well find out early.) They were short staffed so her shifts increased and she ended up being paid. Plus she gained first hand experience of the important but low paid support work that keeps the health sector going. Some of the more academic medical schools seem to put a lower emphasis on volunteering, though you need to have done some.
Use next summer to do some shadowing. The amount of time does not matter, the understanding of what being a doctor might entail, does. Also look at outreach schemes run by nearby medical schools, and work like blazes on UKCAT and BMAT. UKCAT is all about timing. Regular practice will help, and a good UKCAT score will open up more options than a biology A level. (This one is from bitter experience!)
Also he should retain an interest in biology by reading popular books, say on the double helix, and perhaps could get subscriptions to the main science magazines for Christmas. My guess is that an interviewer might want to probe whether his lack of biology was as a result of a lack of interest in biology or because of a greater interest in maths/physics. A decent general knowledge should allay fears.
Sorry about the essay. Last year's application was pretty stressful, though DD was lucky enough to end up with two offers. The other thing is to treat medicine as a two year application. If you don't get in first time round, try again, using the year to strengthen your application. One possibility would be to work in a health care environment (either in the UK or abroad) and study for biology A level at the same time, only sitting it, if any offer has that as a condition.