Why hello there Hazel, wishing you a very Royal welcome to planDisney!
I'm so glad that you came along to ask this question; as a thrill ride enthusiast myself, I've been trying to slowly train my nephews into more and more
daring drops and faster rides with every Walt Disney World Resort visit.
I found that a good way to compare is to have a look at examples of water rides similar to the one you mentioned, or attractions that are considered to incorporate "Small Drops". The slight drop near the beginning of
Pirates of the Caribbean may be closest to what your son has already experienced, while rides like
Kali River Rapids and
Frozen Ever After are much higher!
From this list alone, I can understand why the
Guest Services Cast Members might've found it difficult to explain in detail the classification requirements that would merit a "
Small Drop" designation for a particular attraction - they seem to vary significantly based on each ride experience!
Though I wish I had the chance to speak directly with the Imagineers who were responsible for the construction of each ride, I found that the best way to gather information about each one was to go straight to the friendly Cast Members operating the ride themselves and inquire about the relative heights that guests might expect on each drop.
From their best estimates, it seemed that a drop of 25 - 30 feet or higher would be considered a "Big Drop", as in The Twilight Zone Tower of Terror and Expedition Everest - Legend of the Forbidden Mountain. Anything less than that might be generously classified as a "Small Drop", which ranges from screen-based attractions like Soarin' Around the World that may just simulate the feeling of a swooping drop to the gentle physical dips of Peter Pan's Flight.
Unfortunately there appears to be no real set of metrics available to guests that compare the degree of each "Small Drop". Instead, what I might recommend is viewing on-ride videos of each attraction online to gain a better sense of what its "drop" might actually entail before you ride it in person.
Please feel free to come back to planDisney to
ask me if you have any other ride-related questions, or anything that may be helpful for your next visit!
May your upcoming adventure take you to new heights and thanks for "Dropping by"!
Prince