Apparently in the UK, all A-Levels certificate for each subject is independent, and not in one single form.
So they can handle it with ease, if they want to retake any subs.

For TS Case, I think it is better for him to take 3 subjects.
As that's the UK Standard.

So he can complete the 3rd subject, after he completed his first 2 Cambridge A-Level exams.
Some UK Sixth Forms, People do only 2 AS+A2, and 1 AS, because they felt they have no interest in completing it.

That AS certificate alone, is now an independent certificate, only in other boards than Cambridge.

Meanwhile for Cambridge A-Levels, you need AS and A2 to get an overall A-Level results.
Your A2 results will never become an independent certificate.