The test is a computerised test which, apparently, cannot be tutored for.
DS got a conditional offer and can't remember a thing about it however the interview is of key importance and a boys other talents and his personality. It is rare for them to pick a boy who is only academic, unless his test results are unusually high. Your son will also have to be fairly fluent in English.
Eton do not just take the top 250 based on exam results. In fact they only take the top 30% and the rest is based on a number of qualities as mentioned above so in theory a boy who got the lowest academic marks could get a place.