Generally, someone will "play" with him and assess his skills, at that time they're not going to want you to get involved at all. If they push him to do something that he can't or won't do, just let them try because they're trying to provoke a response from so they can see his reactions. Someone will probably also ask you a lot of questions about his daily schedule, there will be a lot of questions like "can he kick a ball?" " "how many words does he use?" - things like that.
Bear in mind that no parent knows exactly what they're getting into when they go for testing so the people there are prepared to help you all the way through it. Don't let yourself get too worked up anticipating it. Just let them observe him without interfering and try to answer all if their questions as honestly as possible and you'll do fine :)

I also filled out lots of questionnaires regarding behaviors and their frequency.

According the book Parenting Your Asperger Child, it is important to get a speech evaluation as well. Are you getting a cognitive evaluation as well by a psychiatrist? This book and others can guide you. Even though my son at age 8 got a diagnosis of ADD from an Autism Clinic neurologist- when he went for social skills training and writing skills training with OTs, they got a diagnosis of autism disorder from the same neurologist who oversees other clients of theirs.