I personally liked the idea of going from first to sixth, and having all those priveliges and different uniform I'd seen since I was 11 - although I do remember looking around at the time. I'm glad I stayed, just because I'm a bit of a sentimentalist and like continuity I think!
I quite hope dd (now in year 10 so we'll be thinking more seriously in the Autumn) stays where she is, as the sixth form is quite new and I'd like to support it) - but we'll look around at the other sixths in the city too. Many year 11s do move round here, either because their original school was 11-16, or just because they want a change, or to go to college - all the ones I've known to do so have been pretty confident in their decision and seemed happy, although I think inevitably the first few weeks are a little daunting. Certainly where I worked, you couldn't really tell by Christmas who'd been there since September and who since year 7.
I think probably if he feels it's the right thing, it is - two more years in a place you feel you've outgrown could be quite tiresome.
Bear in mind though, that he might change his mind - I was dead keen to go somewhere else almost up the last minute, and all about the change and casting off the old and all that, and then thought noooo, I want to stay where I am!