C.C.
I think you're right to be concerned. Especially when children are very young, or otherwise unable to tell you what's going on, you should be able to drop in at any time unannounced. How else will you know how they are interacting with your child? I was shocked to drop by my youngest child's daycare when she was about 4 months old, to discover her teacher in the next room SMOKING (!!!), and the babies left unattended in swings. Needless to say, that was the last day we attended that daycare. How would I have found out that information if I had made an appointment first?
Given that your son is autistic, it may be a bit more difficult for you just to change preschools, since I'm sure a good program is hard to come by. In that case, maybe you can still drop in anyway, by bringing items that you "forgot" to send with him to school. (a sweater or his snack or whatever.)