Ummaadam My DD is at the end of year 12.
I agree with others on finding out the exact specification that your DS will take. The specifications can change often so you may can only go back as long as that specification is in effect, as otherwise you are using 'legacy' papers for that subject with a particular board and these may include additional material which is irrelevant or may miss topics out.
As others say, you can download these papers from the relevant website. However, more recent papers are less likely to be available. You can buy these recent papers direct from the exam board - with OCR last year I phoned up and gave my details over the phone and they sent them to me very quickly.
In my experience, schools tend to use the most recent paper for the mock - not always but very often, as this is almost always a 'secure download' and so is unavailable online, and is not available direct from the exam board for several weeks. I agree with others, it is much better to do a mock 'cold' so you have an accurate assessment of how your DS is performing rather than him doing a paper he has done before, so I would advise you to not to buy this one until after the mocks.
DD has told me that she believes the reason she did well at GCSE was because she had practised so many past papers. I think your method of grouping the questions is very sound, but it is going to be very time consuming on your part. Also, many questions take in several topics in one question so some would be difficult to categorise.
Your method of learning a topic thoroughly and then doing all the questions on that topic is very sound and I think in an ideal world would work perfectly. The problem is though that this method may just take too long, especially considering the sheer number of GCSEs they are expected to take now. DD found that one of the problems she encountered with GCSE was retaining the volume of information on each subject. She would often say she would learn a new topic and then feel that on old topic had 'fallen out' of her brain. I honestly don't think your DS will have enough time for this approach.
With this in mind, you could use the papers more efficiently by getting DS to learn a topic and then try a couple of exam questions for that topic. Do this for each topic, and then when he feels fairly confident with all the topics, then try a complete paper. In this way, he will be keeping more on the information at the front of his mind.
Don't worry about the timings of the papers to begin with, it is more important for him to understand what is required. Gradually work up to doing them in the required time nearer the exams.
As far as revision guides are concerned, our school was not very helpful. I bought the CGP guides from amazon and they were good - lots of simple explanations and extra questions.
Good luck - hope that helps a little.