Hi wahwahwah,
Maths, reading age, verbal reasoning score all figure highly in selection.
If your son is really bright and inquisitive he will do well but competition is fierce and parts of the selection process subtle. All candidates are closely observed at all times by one or more staff (even during the relaxed playground session). Do they get on with others? - they have been given team tasks to do in past years. Do they seem to enjoy being in the school? Do they ask relevant and intelligent questions? Can they hold a conversation with an adult. Are they verbally dextrous? Do they 'understand'?
Coaching? Always easy to spot during selection. Strangely, some very able candidates who don't really need it get coached nowadays possibly because of parental neuroses. Some practice at maths and VR tests will help create familiarity with the format & build confidence (esp for those from primary state schools), and they'll need to be reading fluently well above their chronological age, ditto spelling.
Talent is easy to unearth (as well as the sheep in wolf's clothing, if you follow my drift) and is difficult to manufacture artificially.
Parents need to understand that there is a suitable school for their child but it may not be WUS. 'Marginal' candidates accepted into the school have struggled in previous years but gone on to flourish at other prep schools. They have to be right for the school and the school has to be right for them. Parental ambition ('I want the best...') has led to many a tear at the school gate once this lack of congruency kicks in.
Finally, parents need to understand that if their son does get in he will need to be supported at home once he starts. Homework is challenging, frequent and unrelenting; he will be examined probably more often than is good for him and require proper supervision, time (& your time) + a proper space at home to work to cope with all that. You will be paying the fees of course but that does not absolve you of the practical aspects of responsibility for his education (although one or two do think that).
All of the above sounds terrible but once happily settled in the school they flourish, have a real thirst for scholarship (in the broadest sense of the term) and ideas in general. Almost all are extremely happy.
Hope this helps.