Myself, I put him in Ravenclaw. His whole life is focused around books and thoughts, and he's not comfortable with actions, counting out Gryffindor. He's definitely not loyal, so out the window with Hufflepuff. Slytherin is a possibility--there is that ambition thing going on--but I opted against it when I Sorted him for two reasons. The first is just the whole unlikeable character=Slytherin cliché that I wanted to avoid; the second is that he not only shows no hesitancy around Muggle-borns, he actively favors Hermione (the brain, which would also be something a Ravenclaw would consider praiseworthy... even if it was only for her ability to memorize his own books).