But let's follow him to the end: how can he claim that we ought to behave as if there were such a thing as morality?
Obviously, it doesn't make any sense to ask what we ought, in a moral sense, to do, having concluded that there is no such thing as morality. But that's not what Joyce is asking. He's asking what we ought, in a practical sense, to do.
One possible answer is, "Jettison the moral discourse entirely." Joyce writes,
For all I know, "Jettison the discourse" is the correct answer.... However, I do not think that it is the only candidate....The reason is that acting according to a moral code might actually, for the most part, be beneficial in a practical sense for a given group of people. But those people cannot simply decide to believe in such a code - not, that is, if they have already concluded that morality doesn't exist. However, they can decide to act as if their moral code were real, act as if there were such things as categorical imperatives. In doing so, they are able to reap the benefits of a moral code without committing the logical error involved in accepting categorical imperatives as actual. This is what Joyce calls the "fictionalist stance."
The fictionalist thinks the correct answer is "Keep using the discourse, but do not believe it."
Joyce admits a problem with the fictionalist stance, namely, that we can't adopt it among people who actually believe in morality. To do so would be dishonest: we would be using the same terms, but in a different way. The alternative would be to preface any moral statement with some sort of disclaimer, to the effect, "I'm going to be talking as if I believe in morality even though I actually don't." One can easily imagine how effective this would be.
So, by his own admission, Joyce's solution only works if a group of Joycean fictionalists went off to an island somewhere and all agreed to adopt the fictionalist stance. Clearly, an impractical solution!
Worse, just imagine the kind of moral discourse that would take place on this island. "You really ought not (nudge, nudge, wink, wink) to rape that child."
I think there is a better response than this - which I'll try to sketch next time.