I assume that what you mean by "whatever he says it is" is whatever preferences his choices reveal, not literally what he says it is.
Believing that a person's good is literally what they say it is can just as easily lead to "nannyism", if we decided to prevent people from acting against their own good.
It's a balance, what with akrasia and all - but yes, flat out accepting that people want precisely and only what they verbally and publicly indicate would be problematic.
Here's the new thread for posting quotes, with the usual rules: