Being told something will be cool wouldn't really have reassured me. Especially if somebody told me it would be cool because of "all the new friends". Yipe! That would make me want to hide somewhere.

The problem with a new school is that it is new. The best way to deal with new things is to make them familiar. Can you visit the school? Tour it? Maybe more than once--even sit in on lessons? Let him know where he'll eat lunch, where he'll have his classes. If he's going by bus, he could see the bus. If you can't do that, photos are a poor substitute, but they're still better than nothing. It will be easier to deal with all the new people if he at least knows roughly what to expect.

Some auties are actually very sensitive to the feelings of those around them, instead of being oblivious. If your son is one of those, he'll do best if you yourself are calm about it and see changing schools as something rather unremarkable, perhaps a little annoying or interesting, but not particularly overwhelming.