Thank Olga 87. I think you are right. His class has one teacher and several helpers. The helpers are all students. They stay for several hours each time. Some of them are nice, some of them look reluctant. And I saw once one helper just ignored my son when my son ask something from her at least 4 times when I picked up my son (he did not peak English, so she might not understand him).
I better change to another daycare and see if he does the same thing in the new place.