Falling Star gave some good advice.  I, too, have a son who is mildly autistic, and when you first posted, that was the first thing I thought of, b/c it is such a common response with our kids.
DS, who is a twin and 4, has begun telling me what rides he does not want to do.  I let him talk, but I do not indulge his fear.  Showing videos is a great idea.  Alot of our kids are visual learners and like to know what to expect, and are not good on spontaneity.
Last trip (May) I let him say No, to rides like Splash Mountain.  However, in Oct, I will not.  I will prepare him the best I can ahead of time, but I also know, if he tries the ride, he will LOVE the music, and love the animal scenes.
I am actively involved in autism groups in my community and know some parents who "protect" their children from every possible fear.  My opinion on that is that I am trying to encourage bravery, self-esteem/self-efficacy and courage from him in life.  There are no guarantees in life, and although I would like to always be there for him, I don't know for sure if God has that same plan.  So, I try to prepare him to believe in himself and face his fears.  He has always done well.  I realize these are only my opinions, and I'm not trying to tell anyone what to do.  But with "our" kids, I am finding that my approach works very consistently.