Have you explained to him that it takes longer for him to sloppily do his work and then have to redo it more neatly than it does for him to do it neatly the first time? That would play on his desire to have more free time. The other thing I would think about is talking with his teacher about a trial period where she gives him more difficult work to see if this would persuade him to work more neatly. If the more challenging work is interesting to him, you could always tell him that if he wants to continue with the more interesting work he will have to show the teacher that he can do it neatly. And by neat I mean kid-neat not adult-neat.
