OP - an independent prep will not necessarily prepare your DS for a state 11 plus exam. Please do check out the format for the exam. Often grammar school entrance exams consist of VR and NVR only, whereas the prep school will be preparing your child for the format of whatever independent seniors it “feeds” in your area - ie. there is generally a maths paper, a comprehension paper and they need to know how to write an effective short story in 30 mins or so. They are also prepped in interview technique which independent schools put a lot of emphasis on (whereas grammars may not interview at all - it’s just the top scorers in computer-marked multi-choice tests).
Maybe it’s different where you live, but in London, it’s boys who move into Year 3 or Year 4 (following a competitive 7+ or 8+ Selection process). This is called moving from pre-prep into prep. The boys independent preps are for 7-13 because 13 was the age many traditionally boarded following Common Entrance.
So do check if it’s a boys prep that they even prepare for 11+ because CE is different again and you don’t move until year 9.
We have girls and boys, so they all went to a Prep that prepared them for 11 plus to London Day Schools. Read some of the 11 plus threads on the Secondary Education Board to get an idea. It’s super competitive and stressful - often the odds of a place are 14 to 1. Lots if very bright children who could easily cope in the schools will not get places, even after going through several rounds of exams and an interview for a particular school. They sit maybe 5 schools each - all with different assessment criteria. There are a few grammar schools which take out if catchment (eg Tiffin), but very few move there from the preps at 11. It’s a different exam and once you’re in the independent system, it’s hard to step off the wagon iyswim. For the grammars, it can be just as crazy with state school parents going all click and dagger and hiring specific tutors from about year 4!
I agree, you have no can have idea what sort of school will suit your son yet. There are so many variables!