Slughorn didn't have Ron's name right throughout the book. He wasn't in his field of view so he didn't care to properly learn Ron's name I think.
Good point. It might have been more obvious if he called him "Don" or something. It was just too weird that he called him the name of the actor that plays him in the movies.
Oh! That reminds me. We saw "Charlie and the Chocolate Factory" this afternoon and there is a very short preview for "Goblet of Fire".