N., I'm a pre-school teacher & let me tell you almost every mom I know have felt sad to leave their child at school. The transition time is the hardest. If possible, before your son starts take him to see the school...play with kids & just check out the new surrounding. Also, it would be a good time for you to chat with teachers, watch your child interact with others, & just get afeel for the school.
On your both first day of pre-school (yes, it is your first day as well since you'll be giving him to someone else to care for him in your absence), don't make the good-bye long. It will be harder on BOTH of you. Give yourself a time limit about 10 minutes...introduce him to his new surroundings. When you leave, tell him you love him that you'll miss him. Let him know you'll be back around what time (& teachers too, so they can help him know when that is in his day). DEFINITELY, say good-bye...a hug, a kiss, or however you communicate you love him (some parents come up with special ways like sign language I love you as they drive/walk away).
The first month, he'll have his good days & sad days (don't we all). After that month, you'll really see how you feels & thinks about the school. Monday is always the hardest for them to come, but usually the parents have a hard time getting the child to leave. I hope this helps. I'll pray that the transition will be good on all of you! M~