Sorry, cannot type Chinese.
He should stay at DC only one hour today. But my mother was mistaken and picked him four hours later...
He cried a little, but did much better than yesterday (when my mother stayed the whole hour with him in DC). My mother asked him whether he had a good time, positive response. Whether he wants to go to DC tomorrow, again, positive response.
According to CY's posting, I think my son is "READY" for DC, so the adjustment is much less painful and drastic. He shows signs of interest of playing with other kids in playground. He plays well with his old relatives. We drop him to his grandparents' house often, so he is use to the idea of not being at home, not being with his primary caretaker.