This is exactly what my daughter did, broke my heart! She just needed to work through this transition. I can tell you what WON'T help:
-don't tell him he just has to do it or just has to be 'a big boy'
-don't focus on anything negative, how much he will miss you or anything hard about school
-don't talk about consequences for crying
what we did with my daughter is that I lined up with her, held her hand in the line, and handed her off to the teacher. this was pre-planned with the teacher, who let my DD be her 'helper'. this way I could tell her in advance what exactly would happen and then it happened, no surprises. gave her a big kiss, told her I loved her and couldn't wait to hear about her fun day.
my advice is to ask the teacher what you ALL can do to help your son out. this way the teacher knows you are open to suggestions and are hoping for her help as well. if her suggestion is mean or just not ok with you, say you aren't comfortable with that and does she have any other suggestions.
it's also helpful to give your son something to focus besides just the moment of leaving you. maybe this is part of what you could discuss with the teacher. ask her what they will do on a particular day (in advance) and let him know at breakfast "today Mrs. X told me you get to color the letter A, what color do you think you want to use? will you do a super job and show me when I pick you up?" now he has a project and something to think of other than leaving you.
Any good teacher is used to this problem and should be able to work with you. and you said he's the only one, so she should have the time to help you out (rather than if she was dealing with 10 kids who are having a hard time). we lucked out with a teacher who was patient and flexible, hopefully you do, too.
BTW, my daughter is now 11 and fine! but I still remember how hard that time was, and even a few moms I've known since then remember her sad little tears. you know he'll get past it, he just needs a little understanding. so do you!! :-)