As others have indicated, every child is different. Ours is at one extreme. She has experienced every WDW, DL and Universal attraction as soon as she was tall enough. She never had a problem with any. She reached 40" when she was a couple of months short of three and a half, and has been riding Tower of Terror ever since. Same for Dinosaur. She's now about to turn 8 and is waiting patiently to reach 54" to ride Hulk and Dueling Dragons.
Now, her cousin is a year younger and cries hysterically during Voyage of the Little Mermaid. So, she is clearly at the other extreme.
I'm not really bringing any unique advice to the discussion. But, reiterating to leave it up to the kid to decide. Describe it, even preview it yourself if possible. At 9 years old, he'll be inquisitive about it and will understand your description. Our daughter has grown up trying everything from all types of foods (octopus may be her favorite thing right now) to wild attractions. She's developing into quite a worldly young kid, and it's fantastic. Encourage, but don't push has always been my approach. I'm always honest and if I think it might be too much, I let her know. But, in the end, she makes the decision - and it's almost always in favor of trying.