The answer to your question is 'sort of'. Yes, the places are awarded based on the exam results, but at the same time, some schools have catchment areas on top of that and some give preference to children who live in particular postcode areas.
Tiffin boys' has a catchment area of a 14km radius and it's very unlikely anyone who lives further from the school than that would be offered a place. Within the catchment, it goes by the exam score.
Sutton Grammar and Wallington Boys' have a number of places for boys in 'preference postcode' areas - both schools have detailed admissions policies on their websites, so please read those. Wilson's give preference to Sutton children when there is a tie in scores.
If you search the forum, you will find a number of threads with discussions/suggestions re. 11+ preparation, but in a nutshell - yes, you need to buy the relevant books and work through them. (Letts, CGP, First Past The Post series are very good, and I 've listed them in order of their perceived difficulty; also Schoffield and Sims for mental maths). Wide-ranging reading and talking through what has been read are crucial for building up vocabulary and comprehension skills. Mock tests in year 5 will give your DS a chance to experience a test in 'real exam' conditions an will tell you how his results compare to the rest of the cohort. There is a lot of general 11+ information, information about various courses and free sample papers you can download on this website, not just on the forum: http://www.elevenplusexams.co.uk/advice