My son goes to QE Boys. There's no catchment, no sibling places, purely test scores in Maths and English. The Maths is crazily competitive...my son got full marks in his SATs but got only a middling score in the maths. (We only tutored for English, big mistake). It's multiple choice.
About 1 applicant in 10 gets a place, so if you feel confident your son could come 1st in a competition with his class mates, he'd have a chance.
It's pretty traditional and academic (inc Latin), lots of testing, great for sports (rugby, not football). It's very competitive - if you want a top set place (my son does not & that is fine ) you have to work hard. There are some amazing opportunities (e.g. Robotics team take part in international competition). The teachers are very inspirational, many quite young and have very good academic credentials.
There is a bit of lack of diversity. The majority of kids are Asian with super-successful professional parents , many from North West London (Harrow, Wembley). Their culture seems to embrace - and even enjoys - the competition , which many white liberal North Londoners find a bit uncomfortable.
All the boys do well academically but you do have to score above a certain percentage in a number of internal tests in Y10 and Y11 to be offered A level. People make a big fuss about it but it is really very transparent now and they have many chances to make the grade. My son is nearing the end of Y10 and he understands his options well.