Are you *sure* he doesn't have some type of challenge that impacts his ability to express himself through writing? What you've written below sounds very much like my dysgraphic ds (who also has an associated expressive language challenge). His challenges don't "look" like obvious challenges when you watch him do school work, but they can easily be misinterpreted as looking like "laziness".
not working to potential, making silly mistakes
he's not putting onto paper what is in his brain, he hates writing, says it takes too long.
Sorry I'm not much help, but those things would make me wonder. Have you asked him what makes putting his thoughts on paper difficult? What type of "silly mistakes" does he make in school - does it look like he's rushing through things?
Best wishes,
polarbear
