Nice to hear from you again, eco!
It's generally tough for ASD individuals to manage change, or even the thought of change, so it's not too surprising that he's freaking out. I think he may need more than two shadow days, and he may also need an introduction-not-shadow day, just to give him an opportunity for gradually acclimating himself to the idea. Perhaps you can arrange a visit to the campus after hours, when it's mostly adults, or mostly empty, so he can start by experiencing the physical space, at his own pace. You might also consider having him meet an understanding adult before the official shadow day, so he has an on-campus resource to turn to, in the even that he feels overwhelmed by all the newness during the shadow day.
And, of course, continuing to mention, in a low-pressure, casual way, little things he might enjoy about the new school. I'm thinking, leave a little nugget out there, without commentary, let him mentally pick it up and roll it around, and wait.
