@Indyschoolq
If he takes the entrance exam in Year 6 for Year 7 entry, he will be pretty much safe until Year 11. If he works hard and his behaviour is good.
To stay on for A Levels, you need at least 6 x 6's.
At Kings and RGS, seniors would be from Year 7. All 4 of their prep schools finish at Year 6, and they would then move to the seniors for Year 7.
Bowbrook is on the same campus from 3-16 and you would not need to take an entrance exam at any level as it is non selective academically.
If you did want to look at the Prep schools:
RGS The Grange is a large school, with loads going on. Class sizes of 15-18. Lots of sports. Lots of extra curricular.
RGS Springfield is a small school, very small class sizes 5-9 children. Very nurturing and friendly. If you have a shy child, this may be better.
Kings Hawford would be much the same as The Grange. VERY sport focused and way more outdoorsy. If your child loves rugby and is good at it, then this would be slightly better than The Grange.
Kings St Albans is slightly bigger than Springfield but with much the same vibe.
Bowbrook is very small from Reception to Year 3, but from Year 4/5, really picks up with 10-15 in a class. It is a nurturing and warm school with dedicated SEN support and is a wonderful place. It is also significantly cheaper than Kings and RGS and offer sibling discounts.
Many children who start to struggle in Year 4 at The Grange or Hawford (Year 4 is a huge jump) are moved predominantly to Bowbrook.
If you son is bright, sociable, sporty and well behaved, I would go with Kings/RGS. If your son is average academically, isn't fussed on sport, and prefers quieter places, I would go straight for Bowbrook.