I posted this yesterday - thanks very much for all your suggestions.
My son said yesterday, completely out of the blue, "I'll miss my friends but I think I would like to go to the smaller school."
I was dumbstruck and couldn't believe my luck, if only he knew what bribes I was going to offer!
We've arranged a taster day for after Xmas. Fingers crossed he won't change his mind in the meantime.