DS6 is very similar. For him, he'd write very sloppy (almost illegible) when something was easy, homework-wise. We'd make him redo it. When he's "into" something, he writes better than most girls that were in 1st this past year. I was always one to rush through to finish work, and though I was not sloppy, I would miss easy questions, etc. This did bite me in the backside as I got older (especially college). So for us, we do stress the importance for DS to turn in his work, take his time and review (something I was HORRIBLE with), etc. He lost a library book (though I still think he turned it in), and while we paid the fee the first time, he know he'd have to pay it if he did it another time.
Teaching responsibility is just as important as academics, imo.