Agree totally with above suggestions for trying to get at the problem. One thought: perhaps see if the nature of the work has changed? Two things that particularly affected our DCs:
(1) In grade 1, the switch from mostly choosing how to spend all their time, to being told what to do, all the time.
(2) In grade 2, being given tons of worksheets - lots of repetitious drill in math, grammar, etc. For this kid with writing issues, so much make-work wasn't just boring, it actually caused major mental health issues.
You can't help (1), but situations like (2) can be easily fixed - and in our case had huge impact when it was. While many kids would rather be playing or doing something more interesting, at this age I suspect they are rarely lazy, and usually like to please the teacher. So there may be something in the *changed* nature of the work or approach that is causing problems, rather than a general unwillingness to work.
As you mentioned writing issues, could there be major increases in writing expectations that are suddenly flinging your DS from years in the "way too easy" over to the "I can't do this" zone? Been there too...
