The school's main job is to get them onto the "next stage", eg through Common Entrance. This is what parents expect and pay for.
Pushing for great results, and nurturing quieter kids, dosen't really go hand in hand. Some sail through without noticing the pressure whilst others can end up quite anxious. Boys at this age can be very competitive with each other to the extent of being unkind, which does not help.
One advantage of this sort of school, and I too suspect I know the one you are talking about, is that so many activities are offered that your son ought to be able to find something he is good at, whether art, drama, or a particular sport, which he can make his thing. However if he really finds it difficult to put himself forward, he can end up being trampled upon.
Individual members of staff may be more sympathetic than you expect. I suspect meeting the demands of both CE and pushy parents can be quite difficult. Perhaps a meeting with a tutor or form teacher to discuss a strategy whereby your son can find areas in which to shine and to develop self esteem?
Otherwise and others have said - wrong school for your child. Children are not all the same and only some will thrive in a hot-house prep. It does not mean that he is not bright enough, nor that he has failed. If you decide to move you should not find it too difficult. Though London parents seem to have a ranking system largely based on how hard it is to get in, and what the end results are, gentler schools often believe that they provide a better and more rounded education. It also suits some children to be towards the top of their year group.
I can think of two sets of parents who pulled their kids out of high pressure preps. One said they got their child back. He had previously become so anxious they did not recognise him. The second boy commented at the end of his first week that his new school was great. People, including teachers and fellow pupils, were nice to each other.
In fairness plenty of boys thrive in a high pressure environment. It really depends on the child.