My son likes our 6 flags "mine train" roller coaster - it's hilly and definitely a real coaster, but no loops and not crazy fast; has a lap bar, not over-the-shoulder kind. At Disney this fall he was almost 7 and LOVED Big Thunder Mountain and 7 Dwarfs, did not want to try Space Mountain or Splash Mountain. I think he didn't like trying something you can't see at all before trying it (except for the big hill/drop on Splash, so all he could see was the "scary" part). Many people report that
youtube videos of the rides are a great way to let them preview and choose.
At Epcot, he loved Test Track and lists it as his favorite in all parks combined. At AK, he didn't want to do Everest - that mountain looks pretty high when you're watching and after hearing all the riders' screams, he was content to wait it out! He did like the Primeval Whirl and loved playing in the Boneyard play area while others in our party went on Dinosaur.
At DHS he didn't want to do Rockin Roller Coaster because it goes upside down; his big sister agreed that he'd probably think it was a bit too wild. Hopefully you'll go to MK before some of the other parks, because its rides are great testing ground for what else he'd like. If he likes Haunted Mansion at MK and isn't at all creeped out by it, he may be able to do Tower of Terror. On that ride, I think all the dropping is the craziest part, but both of my daughters (9 and 13) think that part of the ride is just plain fun, but all the creepy theming was a little too scary for them. If he likes the coasters at MK, it gives you an idea what you might try at the other parks.
Those are my assessments of the thrill rides - other than that, my 6 year old liked every single dark ride, "kiddie ride" and show we did. I don't think there was a single thing there he didn't enjoy.
It's not a ride, but a major signature event of our trip was participating in the Jedi Training Academy at Hollywood studios. If he's at all interested in star wars, I highly recommend it. There's a bit of a science to signing up for it, though, so do a search here or post a new questions if you're interested in more details on it.