Most preschools have a gradual transition-in program where you stay with your child for a little while the first day, leave them a little longer after that and gradually work your way up to leaving them there the whole time. Does yours offer that? that would help ease both of you into it.
Are parents always welcome to pop in any time and see what is going on? Do they provide daily write-ups about your kid's day?
All that said, it is perfectly normal to be anxious and freaked out and a little sad about the idea of being away from your child the first time. I think every parent goes through that. If it's a quality pre-school setting the kids tend to adjust much quicker than the parents.
Also don't be put off if your child clings or cries when you drop him off, and don't take that as being indicative of the whole experience. Some kids have trouble with transitions. My daughter is 5 and she cried a little and pouted and clung when I dropped her off at daycare every.single.day for 4 1/2 years. And then would not want to go home when I picked her up in the evening because she was having so much fun. It's normal for kids to cry a little bit and then calm down almost instantly after the parent leaves and have a great time. So if your son does cry, talk to the teachers and find out what is going on the rest of the day when you are not there.
To Trenell, MizKerri and geeky:
I pray none of you ever has to live in a communist state.
Geeky is my hero. She's the true badass. The badass who doesn't even need to be a badass. There aren't enough O's in cool to describe her.