Dear M. C:
Call the preschool and ask how long he cries after you leave. Chances are, it doesn't last long after you leave.
In July, my nearly 4 son started a new preschool after 2 years at a different school. Because I consider him pretty "well-adjusted" (and had my own opinion of what that meant), I was very surprised at how emotional it was for him to leave his familiar friends, teachers, and traditions and begin again at a new school. There was alot of emotional behavior: crying at drop-off; crying the night before as he thought about drop-off; conversations which began, "Mom, let's talk about [insert name of new school here]; and "one particular throw-self-on-sidewalk-kicking-and-crying incident (with teacher whispering, "Just walk away. Walk away. WALK AWAY.). I gave him alot of room and space to talk about what he didn't like about the new school and what he missed about the old school.
And here's the thing. He had fun at this new school. He was making friends. His teachers were nice. He liked walking to school. He took swimming lessons and field trips. He was really enjoying himself. I was baffled.
Then, one day, a dear friend asked him how he liked his new school. With the biggest smile ever on his face, he announced "I don't like it." And I realized, with all these changes, he just needed to be in control of *something.* That *something* for him was how he talked about school. It was as if he was saying, "You can make me go to a new school school, but you can't control what I think about it."
That was a breakthrough. It took him two months before he would say, "I like [insert name of school here]. So hang in there, and let him work through this big change in his life. Good luck!