C.,
If it were me, I would first take him to the place of where he will go to preschool and just simply walk around and let him see it. I would say to him, some kids your age get to come here and play and learn. Is that something you would like to do? Show him all the fun things that they get to do and let him meet his teacher if possible, but let her know that he doesn't know that he'll be coming yet, so just to speak to him as she would anyone, greeting him and saying to him that she sure would like to see him again.
I would then, gradually ask him what he thought of the fun place that you took him to see. Peak an interest in him and then lead into saying, if you really would like to go there again, mommy will let you stay there for a little while and then come back and pick you up while you play with your new friends.
Then I would tell him that Mommy will be doing work while he is having fun with his new friends and then, when he is done, you will pick him up and go back home (or whatever your plans are for picking him up).
Let him be a part of the planning and picking out supplies. Let him feel as if it were his decision too, so that he feels an ownership of it and will be much more willing to go.
I do (volunteer) work on my computer and my recently turned 4 year old tells my 9 year old, "Mommy is working right now, so let's just play". She totally understands the concept of work. Sometimes she will say, "Can I see your work?" and I show her what I'm doing and she says, "that's cool mom, good job!" She went to 3k last year and will go to 4k this year. She never cried once and even told me on the first day, "it's ok mom, you can go now, I'll be fine." And she was, all year long!
God Bless You,
Take Care,
T. (mother of 4)