The meetings can be so full of information, legal words, laws, etc., that you might find your head spinning a little.
My suggestion, what I did at my daughter's 504 plan meetings, is to be organized. Have your son's medical information (copy of doctor's letters, test results, anything pertinent) in a readily available and organized file.
Have another file with your questions. Write them down as they occur to you before the meeting. Refer to them during the meeting to make sure each one is answered. Ask how often a review will take place, and what steps to take if one of the accommodations isn't working well enough or perhaps is no longer necessary. Who do you contact?
A third file should contain your criteria, and your recommendations as to exactly what will help your son. Think about this beforehand as well. Picture your son at school during the day. How often will he need to take breaks for stress relief? How long of a break will he need (5 minutes? 15 minutes?) and where will these breaks occur? Would it help if he took tests privately, away from the class in a resource room, or would he prefer to stay with the others in the class? They'll ask you a lot of questions, going over all kinds of accommodations, and some will apply and some won't be necessary. But if you have your ideas and questions written down, you won't forget any of them and you'll be able to contribute your valuable insight to the meeting.
I think that most schools want to help their students. What helps most is an organized, respectful, thorough and prepared parent. Go into the meeting knowing that you are ALL there to help your son have a successful education experience.