AFAIK the aptitude tests are largely computer based and really you have it or you don't.
One has to ask though, why is he limiting himself tp WSOp unless he does not plan to take any A-levels?
As a straight GCSE man he will be in competition with a fair number of more academically qualified applicants. That said, a 1st class degree in underwater basket weaving or mathematics at Cambridge does not mean an automatic acceptance.