D., I can totally relate to your situation! My daughter is 5 and had horrible separation anxiety. She went with me to the Mother's Day Out that I work at for 3 years ( in a different classroom than the one I teach in) and this year she attended the four year old program at our public school but she still had some rough days although it finally got better toward the end of the year. I think she cried every single time I dropped her off at her room at MDO (for 3 years!)but since I was in the same building I had the advantage of peeking in on her and guess what...she calmed down every time and had a great day! Her teachers told me that she only cried for a few minutes before she decided to get on with her day and have a good time but it was nice to be able to verify this for myself.
Having said all that here are a few things you can do to make it easier on both of you. Definitely talk about the school and visit it before he starts. Keep drop offs quick and simple...a kiss goodbye, reassurance that you are returning and then hand him over to the teacher. I can not stress this enough! Do not show any hesitation because he will definitely pick up on this. Also, if he gets upset do not give in and take him back home with you because once you do that you have entered into an area that is almost impossible to get out of and you aren't doing him any favors. Talk with the teachers and express your concerns and set a time limit of how long you will let him cry before they call you. I know even a few minutes is tough for us moms to take! If you haven't heard from the teacher in this time limit you know he has settled in and you can breathe a little easier. This transition may take several weeks but it will get better. Be prepared for tears of your own because it is heartbreaking to hand your child over to someone else no matter how nice and trustworthy they are.
Find a keychain or something like that where you can place a photo of your family in it so he can look at it during the day...I am always amazed at how well this works with the children in my preschool class.
Also, some schools will work with you and let you gradually increase the hours that he spends there. Maybe you can take him in the morning and then pick him up before lunch and then continue extending his time there until he can do the whole day. I would use this method as a last resort though but it may be an option to look at before you decide to completely toss out the whole school idea.
Your son is lucky to have such a caring mother! Good luck and remember to let us know how it goes.