Loads of excellent advice in that document. I think applying to medicine is one area where having a supportive adult or two is really useful, I am generally a 'let them get on with it' person but I think taking an active interest as a sounding board has really helped. This is why sometimes I will write 'we'. The decisions though have all been from dd.
At this stage I would think about whether going to the open days is important for your ds. Dd definitely wanted to have seen the place and go to the talk - it is expensive though so you might want to narrow the list a little now and try to see some over the autumn as well as the summer. You will find that some open days clash, the 10th of September is insanely busy as an open day. There are generally also online recordings as a legacy from covid years which are really useful for sifting out places.
We discussed what dd wanted in a uni town - they are there for five years so location is important and a useful way to weed out places. We quickly ruled out London for example. She also wanted early patient contact so ruled out Oxbridge. I am guessing from the A* that you are possibly not in Scotland, we ruled Scotland out because it is harder for someone outside of Scotland to get into medicine in the places she liked due to fewer RUK (rest of UK) places. We also looked at teaching style, disection/ prosection/ virtal anatomy, all detailed on GANFYD's document.
We ended up with a list of about ten places she wanted to see. I think you could easily aim for less than that, maybe six. I would though recommend having a few lower UCAT/BMAT options in there so if they go badly (and you don't know BMAT when you apply) it doesn't seem like the end of the world. If you can see a few of the nearer ones on your list this year, it will ease the process next year. Try not to get too attached to any one university because if he is applying strategically there is absolutely no point in applying somewhere he won't get an interview.
Another thing that I have done is lurk on The Student Room where GANFYD and the other volunteers field no end of (sometimes silly) questions from prospective students. Dd found it all quite hectic, but I guess being one step removed I find it easier and I draw her attention to important posts.
Obviously also focusing on the A levels (or equivalent) is really important, especially for those unis who use predicted grades in determining interview invites. A few reduce the offer if he has an EPQ. Biology and Chemistry are the most useful, only Cambridge I think want maths so unless he loves maths go for something he can get an A in. Only do three A levels, very few (think only QMUL) give any credit for doing more A levels or an AS. Ultimately see it as at least a two year process to include plans for a gap year, then if he gets an offer in yr13 it is a bonus. It is much easier though to repeat the UCAT/BMAT than A levels so they need to be a priority. Do not do another degree with a plan to do graduate medicine, it is more competitive and more expensive. There are quite a few unis which take students with resits but sometimes they ask for higher grades.
Volunteering and work experience is great but try to get a variety and don't overcommit.
Finally if he is deciding between medicine and something else the general advice seems to be to do the something else. Also buy lots of wrinkle cream and hair dye for yourself!