Shegot...your son has a point. DC went to a (relatively) relaxed London prep. I liked it when the science teacher, who the children loved, suggested it did not really matter what he taught them in Yr4, as long as they acquired a love for science. So lots of pops and bangs, though in fact DD learnt a lot and was still repeating stuff she had learned in Prep all the way up to Yr 10. She is now taking science A levels.
The problem was 11+. They did very little preparation as the school went up to 13 and the teaching continued through year 6. This did not matter for several destination schools who seemed to factor this in in the way they would factor in preparation differences between other private and state schools. The weird one was G&L which in my daughter's day had not taken a single pupil for about five years, but regularly seemed to take mini bus loads from the more focussed Central London preps.
It worked out fine. DDs experience, common to many, was that she was some way ahead when she started senior school, which helped her believe that she was one of the clever ones, even if it was not strictly true. (I suspect it is pretty easy to get lost during Yr 7, and if you start as a middle set kid, that is where you see yourself.) She was also used to taking part in drama and expected to turn up if picked for a school teams, and indeed expected to win.
The parents who were very focussed on the very academic West London schools would panic in about Yr 5, moan a bit and then get in tutors to help with the VR/NVR prep which usually worked out if the kids was bright enough.
The real impact seems to be at sixth form level. Each year a surprising number seem to switch to the highly selective sixth form my son attended. And when there to fully engage in music, sport and leadership. Very unlike bobo's experience "that the last 4/5 years of any school are pretty focused on standardised exams". My daughter's peers also seem to have made their individual marks at a range of other schools. She used to bump into them regularly at matches and now a significant number seem to hold senior Prefect type positions.
I think both you and your son are right. 7-11 are important as this is the time you learn how to learn. The biggest advantage for me was that both DC learned to balance academic and non academic stuff and do their homework quickly and efficiently at the after school club, so procrastination was never a problem at secondary. 11-16 are important as this is when children find themselves, their sense of purpose and who they are, as well as setting the building blocks for the future.
I think sixth form is also important. A good experience should mean that a young adult enters University with the wider leadership, social, resiliance and other skills that will take them through the tough (and University courses are tough) University years, not just coping academically but able to contribute to the wider community. This then should help them in the increasingly tough employment market.
In short Shegot is right, you have a choice. People often focus on results and where pupils go on to. I would suggest you look instead at the pupils in the top year and decide whether you like what you see. Are they purposeful, active and engaged, are they weary and over coached, are they immature and not reflecting their potential? Can you see your child fitting in with this sort of child. Obviously teaching has to be adequet, but the overall education is the key. Results should then follow.
(And I don't think it is just private/state. Private schools normally have more resources to apply to extra-curricular and perhaps fewer children who need a lot of senior management time, but a good state school, would be better than a poor private school.)