回复：谢谢楼上同学分享经验。似乎大多数人倾向于晚上学。谢谢你们给出详细的理由。很多是我们现在想不到的。

I believe at this point, changing your son's grade back  would not give him more than what he already has. He has developed a social loop and he seems to be doing well in it. It may not be natural for him to enjoy a new loop of kids who "seem" to be less mature than him (from his point of view). In other words, it is too late to change:-)