I know others have told you this but, really, don't let him see how nervous you are. He will feel more comfortable if he thinks that you are comfortable with the school.
Tell the teachers what to look for in the event of an allergic reaction. What are his first symptoms? Make sure every adult in the classroom knows. Also, be sure to keep any necessary medications on hand for the teachers to access. My son's teacher has benadryl and an epipen clearly labeled just in case. He's been at the school for a year and a half and we've never had a single exposure and he is allergic to dairy, eggs and nuts (dairy is in the snack almost every day and he's NEVER had a problem).
Expect some tears, but also know that he will probably stop crying and start having fun within a few minutes of you leaving. So even if he's hysterical when you go, don't stay behind and don't stress about it. I know it's hard, but I've seen it so many times in my son's class (it is a co-op, so I'm there a lot, especially in the beginning of the year) and no one has ever cried the whole day.
Ask if you can volunteer in the class once a week so you can observe things for yourself. It'll be a great way to see how the teachers interact with the students.
I don't know anything about pdd, but make sure his teachers understand this disorder and how to help your son manage it. Find out the routine that they follow and how they communicate with parents. Find out how they intervene if two kids are fighting over a toy. Find out what happens if your son doesn't want to do a particular activity.
At the end of each day, talk to the teachers for a couple of minutes to find out how things went and if there were any issues.
Have a wonderful first day and try not to worry too much!!
