I work at a prep school and know the schools you have mentioned well.
Canford harder to get into than Sherborne, but definitely still broad. There are plenty of less academic, mainly boarding, boys, but they tend to have something else to offer - county sport, sailing, serious drama, or be the younger siblings. A few boys I know have taken places at Canford when they’ve been offered both. Don’t know of any the other way around. It also seems to have those that have eschewed Winchester / Marlborough/ Wellington, for various reasons. Very good pastoral care. Nice, sensible bright girls in the mix.
Sherborne has very good music credentials. More focus on the Abbey and tradition. Has beautiful buildings in a nice town. Sherborne has double the boys , so much stronger rugby, but if your son is cricket, rowing or tennis mad, Canford may be a better choice.
Bryanston has been popular with our pupils historically, but seems to be going through a tough patch. I do know hardworking children who’ve done well, but have struggled pastorally.
Dauntsey’s? Good all rounder, less academic than Canford, more so than Sherborne, plenty of military families which mean there’s strong boarding?
Agree Clayesmore now very SEN focussed. Pretty, quite local, not ever been known for sport.
Millfield is one I know less about as the children I know who went there were either very sporty or very dyslexic, so fewer in number.
Taunton good for cricket, but I’d ask exactly who is in at the weekends, what percentage overseas boarders usw. In fact, I’d do that anywhere. Sherborne seems to be relaxing its weekend requirements.
If you need full boarding in the SW, I think you are on the right track with Canford and Sherborne. Just go and visit both again and speak to the pupils. Decide whether you want to prioritise sport or academics. And which sport? Ask about learning support at both. Is your son someone who is determined to keep up if he’s at the bottom of a set, or someone who gives up of he’s not at the top?