He's in the Sox hitters heads at this point, and he is utterly confident. The point of bunting (I mean for base hits, not sacrifices) is to make him do something he hates to do, whcih is field them. Maybe it throws him off a little. There was a game a few years ago when a couple Sox hitters did bunt on him and he was visibly mad about it.
That being said, it's a moot point so far as Red Sox hitters are concerned, because none of them can bunt. Moreland has done it for base hits against the shift a few times over the last couple seasons, that's about it.