We had a similar sort of thing with DS1 but were saved by a game DH had always played with DS. DH had always looked in DS's ears to see what his 'pixies' were doing (followed by descriptions of parties, or spring cleaning, or sitting on deck chairs with newspapers etc.)
Then one day, about 2 weeks before the big day DS sat down with me and told me his pixie wasn't very sure. Oh, said I, what's pixie not sure about? School, said DS. What about school? Whether he'll go! Yikes!
After a bit of both of us counselling 'Pixie' it turns out that he wasn't very sure that he'd be happy. We focused on the smiles on the day of the visit at the end of the summer term, and the toys and activites available there, and the one friend who would be there too.
It worked and the first day was spent planning a garden (DS's passion at the time) with 2 children who became firm friends in the coming weeks.
Is there any way you could engineer a conversation like this with your DS? Puppets are good ways of focussing the attention 'away' from the child and to what might need to be said. What does the puppet think about it?
Good luck. I'm sure he'll be too busy to think much about it once he gets there though it might be worth working out where he's got his info from if it isn't his own experience.