Play it by ear, wantitall. It's possibly way too soon to tell what type of DC your DS will be at 9/10 (when he would need to be doing 11+ prep). If he's competitive though, he's bound to enjoy the challenge as my DS did.
All children (and parents) are different but really it shouldn't be so all-consuming - and unlikely to be if he stays in the state sector until 11 (although rather different in prep schools which seem very focused on being able to show how many scholarships/bursaries/11+ passes their pupils have achieved) - that it seems like lots of extra work.
More and more of the 11+ exams (including those for the Tiffin schools) are moving towards examining children in Maths/English alone (although that's happened since DS took his a few years ago), so the 'extra work' would perhaps just be more focused homework and reading/writing/maths exercises which could be shoe-horned into school-related studies (so it doesn't noticeably seem more!).
As a parent too, you may benefit from not being surrounded by the 'spin' (mentioned upthread) that can really make that whole preparation period rather stressful.
It is also worth considering that the nature of 11+ selection process, catchment areas, ring-fenced places for children in receipt of 'Pupil Premium' and the upward trend demand for places is evolving all the time, so grammar school entrance now is likely to be a very different 'creature' to how it looked a decade ago and how it will look ten years hence.
Best thing to do is keep asking questions on here, on the main 'Education' board and the 11+ Forum.
Out of interest, do most of the Holy Trinity children go to non-selective schools in Merton or private ones or possibly 'faith ones' out-of-borough - assuming the former but find this quite suprising!