R.N.
I know it is so hard. He will get through this. Even kids who have been in daycare since they were infants go through clingy stages where they cry when you leave. A few things that may help:
Read books about going to school/daycare. We had "Preschool with Teddybear" and read it every day for 2 weeks before my son switched from in home care to a daycare center.
Let him know what you and his dad are doing while he is at school. He may think he's missing out on time with you both and it may help to remind him that you are both working and there is noone at home so he goes to school to learn and play until you can all go home and be together.
Try a chart, with stickers for tearless days (you may need to start smaller - (with a sticker for not crying on the way to school, another one for not crying when you leave, another one for not crying all morning at school, another one for not crying all afternoon) And then 10 stickers (or 3 or 5 at first) gets a small prize or special treat (matchbox car or movie with Mom) Try not to set a "pie in the sky" goal for him, he will want to get more stickers if he can see the goal is achievable. Make sure to praise him if he keeps his composure, even for a short time and never take away a sticker or prize he has already earned (even if he throws a fit in the afternoon, you still want to praise him for a good morning). This is a bit of bribery but sometimes we need a bit of that to show that behavior has consequenses (good and bad) when the actual consequenses are not tangible enough for them to understand.
Good luck. Please remember that you are not the only one who has gone through this and that when people see you with a screaming child, they do not think you are a bad mom, anyone who is a parent has been there. Sorry this response is so long. I hope some of this helps!
R.