Without hearing the answers to Kriston's excellent questions, I might throw one possibility out there and see where it lands.
I have noticed with my DS, who was also 7 years old in 3rd grade last year, that it took a little bit of time for him to get used to the mental fatigue that was involved with sitting in a classroom all day long. I'm not necessarily saying that the work it too hard for him... I'm trying instead, without knowing what to call it, to put my finger on the energy needed to sit and be focused for longer periods of time. In third grade, the kids have fewer breaks to get up and move around, and they are expected to concentrate for longer stretches. I have noticed that at the beginning of each year, my DS is just plain tired by the end of the day. My normal little bundle of energy just drags when he is coming home from school. It is not that the material in school is too challenging for him. I just think that the sitting and focusing is mentally draining for him at first. We tend to see that his emotions rise to the surface a little bit easier when he is tired. This doesn't last for too long. He usually adapts quickly to the school environment and is back to his normal bounding self within two or three weeks.
I don't know if this is an issue with your DS or not. Sadness is certainly different than tiredness, but sometime seven year olds can have difficulty separating the two. Heck, my DS is old enough to know that I turn into a cranky grumpasaurus when I am tired.
