Sounds very familiar!
When you drop him off in the morning, make the good-bye loving but short and sweet. Give him a hug and a kiss, promise him that you'll be back later in the afternoon to hear all about his day, and then get the heck out of dodge! The longer you linger there, the worse it is for the kids. Also put on your happy face because if the kid sees you upset, the anxiety will only heighten.
This happened with our son - he would have an absolute meltdown at drop off. We also knew, however, that literally 3 minutes later the crying had stopped and he was playing gleefully (typical of most children at drop off). We created a rock solid routine that we still follow to this day: Once he gets to the door of his room, we pick him up for those last hugs and kisses. Then, we literally hand him off to the teacher for his "Special Good Morning Hugs from Miss <teacher>!" She will pluck him from my arms and give him his special morning hugs. It's now part of the routine!
Maybe talk with the director and/or his teachers to figure out how to troubleshoot. Most of these people have dealt with hundreds, if not thousands of kids during their tenure and I'm pretty sure they've face this before. Form a team, an alliance with them to help your child.
Good luck - I know it can be tough at times :D