First of all, don't be afraid of the huge disruption to the class. That is the teachers concern, don't take on that guilt as well as the Mama Guilt. The teachers SHOULD be sweet and work with you and him, because that is their job. This can't be the first kid to have a hard time adjusting.
Don't let him see you get down or feel guilty. No crying mom. Not until you are safetly in your car where he can't see you. Try to stay positive, I know it's hard, but everything in him is looking to you for clues on how he should be feeling and reacting.
Try to make drop offs short and sweet. Also, could your hubby drop him off? My almost 4 year old hardly even has time to kiss her daddy before she is off playing, but if I do drop off on a random day, she will cling and whine as I try to leave. She knows mom's a sucker ;)
A game I tried was "which way do I go?" pretending to not remember the way to the front door. She thought that was funny and would show me. Distraction is your best friend (but I don't believe in sneaking away without saying goodbye). Also, it sounds mean, but if they feed him breakfast right away, don't feed him (much) at home before you leave, when my daughter was hungry she wanted to go EAT.
I got this book from Amazon, it's just a little story but it helped.
Adam's Daycare by Julie Ovenell-Carter
Hang in there. It gets better.