Some random thoughts... but one thing you mentioned earlier was that his reading level was lower due to writing skills - and you mentioned earlier too that the issue seemed to be tedium. It is probably not related but the more interesting materials are going to be in the higher levels.
Also, does he tend to be one of those kids where even if most of the day was good, if there was one bad moment, it dominates his feelings about school that day?
Last random thought - although the kids are well behaved, do you think he feels there a disconnect between him and the other kids - or like he feels like he is a misfit even though he can interact appropriately with the other kids? We saw major issues with DS even at age 3.5, where every time we watched him in daycare, he seemed to have a blast playing with the other kids, but he would come home and complain how he hated daycare - when we moved him to a gifted class at 4 (another school), all of a sudden, he loved his new friends and the environment. He found a group of kids he could relate to, and talk to - and yet if we watched him with his age peers (at his daycare), you would never know he was feeling like a misfit because he knew how to blend in with his age peers.
