I would question him about why he changed his story, whether Knox left that night or not.
He changed it, then he brought it back to the original version, now it seems pretty clear to me that he's changing it again. To me, it sounds like he's basically saying, I know I didn't do it, but I don't know about anyone else. He is just focusing on his own self. To me, that makes it sound like he is going back to his "load of rubbish" story where he claims he was at home and doesn't know what Amanda was doing.
So now he's changing his story again at this late juncture - very odd.