Those were my AS results exactly a year ago!! I also got a C in General Studies
I decided to drop Maths, it was a pretty easy decision because I didn't really enjoy doing it, and I didn't think I would ever get an A at A2
However, I did retake one unit of it. My three exams were a, c and d. I took the 'd' unit, raised it to an a, which leaves me with a B overall now. Will your son have to retake two units or just one to get an A overall?
I retook that because quite a few medical schools have minimum grades for the AS you drop as well as for the three full A levels. Some want another A grade, others wanted a B and a couple wanted a C. Others didn't have any minimum grade. I knew I didn't have a great chance of an offer (no one does really!) so I felt by getting my 4th AS to a B, I would have a greater choice of medical school next time around if it came to that!
However, this admissions round, there were quite a few medical schools i couldn't apply to because of that C. In the end, i applied to 3, none of which had a minimum grade for the AS level
I got an offer, and am currently waiting for confirmation of my place (not happened yet because i needed an OH assessment first). My interview (for the uni i got in to) was offered on the basis of my UKCAT score (697.5) and predicted A2 grades, and the place was offered on the basis of my interview. Nothing else really mattered at all.
So yes, it's very possible to get a place with less than 4 A grades! My two C's did not matter because I had 3 A grade predictions otherwise, and I applied to the right medical schools. i was surprised by how much admissions information i managed to get hold of actually, and I think the very best chance of getting a place comes with doing lots of research and applying very strategically. Too many people apply to places that they don't stand much chance of getting into
I think if your son gets 3 A grade predictions, he stands as good a chance as anyone else of getting an offer, as long as he applies to the right medical school. The A grade at AS history will allow him a larger choice of places to apply to than i had
Good luck :)