We had this same problem, and to some extent still do. Currently the boys are 7 and 10. Last year when they were 6 and 9 we planned a trip to WDW in Oct (2002). Well, in Summer 2002 we took them to Valleyfair in Shakopee, Minnesota (like a mild Six Flags - it's Cedar Point's little sister park).
All they would do was the bumper cars.
Since short of DQ, Disney doesn't have a bumper car ride, we sat them down and explained to them that we were going to cancel the trip until they were ready to go. None of us were happy to cancel, but after feeling like we'd wasted $80 on a local park, imagine how we would have felt if we'd spent several hundred with the same disappointing results!
So this year we told them both that we would take them to two test parks first, and if they were at least a little brave, we would go to Disneyworld this year.
We took them first to the local carnival. The 10 year old rode EVERYthing, even things I wouldn't do. The 7 year old rode all but the scariest ride there, which is much more than we thought he would do.
So we booked the vacation.
Then two weekends ago we took them to Camp Snoopy, and we wondered if we jumped the gun.
While the 10 year old was still completely fearless, his little brother suddenly wouldn't try anything new, and he wouldn't go on rides he'd enjoyed at 5 years old! We ended up coaxing him and describing rides to him, and trying anything we could think of to get him to try them. Eventually he tried all the ones we asked him to do.
He HATED the log flume (the silent tears thing someone else described.) But he LOVED everything else. He even rode the roller coaster four times in a row. One thing we found worked with him was to withhold some of the minor things from our description. While going into a simulator ride, we told him we were going to watch a movie. We didn't tell him the seats moved, and he didn't hesitate to go in. He loved it, even though he was at first afraid that the seats would go up to the ceiling and fall back down. We assured him that wouldn't happen.
Bottom line: we're going to WDW. He wants to ride Alien Encounter. He doesn't want to ride Splash (go figure, huh?) I'm confident we can at least get him to try Splash, but we'll probably end up taking each ride one at a time and see what he feels like that day.
(Sorry so long; I hope this was helpful)
Jon