What he's feeling: I'm sad to leave my mom and dad and be alone in a strange place.
Your response: Try not to look worried -- kids pick up on that. Remind your child how much fun school will be and that you'll be back to get him a little later. You can also reassure him that his teacher is there to help him and make him feel better.
P.S. He'll stop crying soon after you leave. If you want some reassurance of your own, ask someone at the school to call you later to tell you how he's doing.