I've seen the child's tests where the writing has to be better and it is. I believe that DS knows when he's got to put forth effort and he does - but if he doesn't feel it's necessary, he'll ease up and get sloppy.
I think there's been a mistake -- it appears as though you've ended up with my child.
We added a challenge to his spelling tests starting last year by expecting him to take his time & write neatly. If the teacher couldn't read a word without either squinting or hiring an interpreter, it would be marked incorrect. He vehemently objected (cue the violins), saying how hard it would be & it was no fair & he couldn't do it. And... And... And...
Lo and behold... he still never missed a word. Although, I was convinced he hired one of the other kids to write for him, because I've never seen such glorious penmanship come out of his pencil before.
We raised the bar -- and he was able to push himself to meet the challenge. He had to work at it, of course, but that's the whole idea.
I believe that an expectation is only unreasonable when the child has no chance of success, or, if he would be faced with an inordinate degree of struggle or frustration.
