B.S.
My guess is he just needs to start feeling comfortable.
My girls are the opposite. My oldest had a terrible start to preschool. On one of the first couple days the teacher had to peel her off my leg. It was very hard to leave her and she still is my child of great fear to this day. She just started 1st grade and still isn't a huge fan of school although is a wonderful student.
Now my youngest just started preschool yesterday and couldn't wait to go back today.
I think you need to just give him time. I think the key is that every day he is getting better. Lots of kids cry when they start school. There were a couple in my daughters preschool this year.