Do you leave him with other people at all? If you don't, you may want to get a baby sitter several times to get him used to the idea of being apart.
My DD cried so hard when we dropped her off for Kindergarten. We felt like the worst parents in the world. We peeked in 5 minutes later and she was already smiling and making friends. Teachers know how to distract and focus them on other things. Just don't hang around --- that's the worst thing you could do. Quick goodbye and off you go. Long goodbyes are hard for you, them, and the teachers.
Good luck!
