My DD was pretty darned SLOPPY with her calculations up until she was about 12yo, honestly. It's not that she couldn't do the math-- and in fact, most of the time, it was more like it wasn't worth fully engaging her brain to work through it or something...
This sounds like my DS - unless it's difficult enough to engage him, he gets really sloppy... doodles, scribbles, MESSY numbers of all different sizes, squishes things together so that they're hard to read... (sigh). But if the work is hard enough to keep him thinking, he stops the "pencil shenanigans."
