Yup, chances are, he's going to cry. However, the teachers will be used to new children crying when their moms leave, and they'll know how to comfort him. The key is to give him a hug and a kiss; tell him you love him; and tell him you'll be back to pick him up in the afternoon. Then make a quick departure. The longer you hang around, the more he'll build himself up into separation anxiety. If you leave fairly quickly, the teachers will have a better chance of consoling him and getting him interested in the day's activities.
As for beforehand, I agree that talking to him about preschool and what he can expect will help a great deal. Talk to him about it every day and be very positive. Also, it would help to bring him to the place he'll be staying for a few short visits so he can be familiar with the environment. I wish I'd done that with my daughter!