Sorry you are going through this, and that it is getting you down. I do have to say that I think it is more of a transition issue than a rejection of you. Does your DS have trouble moving from one activity to another, like moving from bath to stories, or something like that? If so, it is probably just something he is working through.
As much as dcc's usually push for a "short drop off," I'm actually kind of a fan of the "long pick up" (depending on how much time I have). If the kids want a few extra minutes, I can sit down on the floor and play with them, or use that time to talk to the teachers about their day, etc. Of course, some people don't have time to allow for this. In that case, I think it is best to just help your child transition as easily as possible, using whatever skills you use at home.
My kids go back and forth on this--one time my four year old even asked me to go drive around the parking lot for "five minutes" so she could keep playing!