M.C.
Be a quiet observer. Write your questions down so that they can be answered at the end of the day. This way you are not breaking the flow of the class or taking the teacher's attention way to answer things as you think of them.
Do not offer to help out unless asked to.
Do not correct or reprimand a child (hard to not do!). You are there to see how the teacher will handle it day to day ... if you do it, you don't get the result you're looking for.
Instruct your son to ask the teacher for any and all help. Again, you are there to just observe. The teacher is the one your child should turn to for things.
One big thing for me was meals. What is the policy for if they offer a breakfast/snack/lunch that your son won't eat?