Thanks very much for your reply!
I looked, and on that particular form, the checkbox is greyed out! Why would it be greyed out? I'm assuming an option setting somewhere, but I can't find anything relevant.
EDIT: Oh, I think I see the problem - he used a template downloaded from Office Online, which for some reason doesn't allow the checkbox in question to be selected.
Is there any way at all to change this? I'd hate to ask him to re-develop his form....