Some good stuff mentioned here OP by other posters, I hope you're finding it useful.
If we're talking Oxbridge, then LRDtheFeministDragon has it spot on - when I was at Oxford 15 years ago one of my tutors burst out laughing at the thought that anyone in admissions would care about extra-curricular activities. They honestly don't.
That said, the beauty of Oxbridge is the interview system, where you get a chance to show what you're about. I can't speak with much authority, but for other universities they have only your grades so far, expected grades, and a personal statement to go on, which is not much to differentiate you from 1000s of other applicants, and therefore having the extra-curriculars does flesh you out a bit. Not sure if it figures much in getting an offer...
My advice on extra-curriculars to you, and your son, would be to do something for two reasons, and probably for not much else:
Because it's fun.
Because time spent doing your extra-curricular is a welcome distraction from revision at exam time.
I also broadly agree with MarytheCanary, although at the risk of gainsaying her, I would dispute that you need to be extremely knowledgeable. In the 4 interviews I had, I was presented with material I had never seen before, and was asked to talk about it (in what was hopefully a thoughtful manner.) Having a nimble mind, and, as Mary says, a demonstrable passion for your subject, are going to be the important elements. If you also know lots about your subject, so much the better, but if you don't, hey, you're there to learn! It'd be pretty boring if you had encountered it all before. That said, Mary makes a good point about having some knowledge outside of exam set texts/syllabus though.
I was not asked 'why this college' in any interview, which was a blessing, as my reasons were feeble.
To address your question about GCSE's (sorry, this answer has become longer than intended), it is always going to be better doing very well in a few exams, especially if they relate to your ALevels/intended university course, than to do OK at a lot of them.