How does he normally act if you leave him w/ a relative or a babysitter? You should be able to predict somewhat how it will go by the way he has acted in the past, if you ever left him w/ anyone.
(My mom was a SAHM and preschool did NOT go smoothly for me at all. But that was a given bc I would become completely undone anytime my mother left me w/ anyone, even someone I knew well. It's usually a little harder for kids w/ of SAHMs...but not always. History is the best predictor.)
Definitely, try to play up how much FUN FUN FUN it will be. Give examples of the fun activities he will be doing and how many nice friends he'll meet.
Has he seen the classroom and his desk and the story area and the playground, etc., already? I would def take him there beforehand and acclimate him to it. Better still, if he can meet the teacher before he is left there.
Let him wear his favorite outfit and promise him his favorite snack or meal or something for afterward.
Best wishes! (Tell us how it goes.)
3b (with 3c tendencies) on modified CG