C.,
First off, don't freak out! It will be fine. Yes, it is inevitable that you let go of your little ones and let them experience the joys and pains of school.
On the first day, you will want to cry, but wait until you leave. Your son will have enough worries of his own, so you don't want to add a tearful mom to the mix. Your job is to keep encouraging him and let him know how much fun he will have and that you will be back shortly to pick him up. Make it very simple for him, without telling him over and over how you will miss him. He needs to feel a sense of calm from you that it's no big deal. And then, when you leave, you can ball your eyes out.
The first few days will be the hardest for you, so make sure you plan some errands and activities to fill your time. If you have nothing to do, you will fill your time worrying about him every minute.
You can let him be involved in picking out his supplies such as a lunchbox, backpack, etc... Let him help you to pack everything. If you will send in pack lunches, let him help pick out his food from the grocery store.
Prepare him ahead that there may be other children who are scared and crying, but he has nothing to be afraid of.
Both of my daughters were very confident on their first days and even told me, "ok mom, I'm fine, you can go now." They waved me out of the class and as I peeked in on them, they were doing just fine. There were others who were crying and clinging to their moms, but mine were very confident and secure. They had great experiences and couldn't say enough about it when I picked them up. I missed them so much, but didn't let them see me too emotional (although it is ok for our kids to see us emotional, this was not the time, nor the place for it). They still love going to school and I can't wait to see them when I pick them up.
You will set the tone for him. Think it through and then, take a deep breath and say a prayer and go.
And then, when you pick him up, be ready to hear about his day and give him your complete attention.
Take Care and God Bless You,
T. (mother of 4)