How am I supposed to argue him out of his current values?
This is a separate question. I was objecting to the relevance of invoking anti-realism in connection with this question, not to the bottom line where that argument pointed.
If moral realism were true, there would be a very obvious path to arguing someone out of their values—argue for the correct values. In my experience, when people want an argument to change their values, they want an argument for what the correct value is, assuming moral realism.
This is a separate question. I was objecting to the relevance of invoking anti-realism in connection with this question, not to the bottom line where that argument pointed.
If moral realism were true, there would be a very obvious path to arguing someone out of their values—argue for the correct values. In my experience, when people want an argument to change their values, they want an argument for what the correct value is, assuming moral realism.
Moral anti-realism certainly complicates things.