The person you talked with is correct.
They put the warnings out and it is up to you to decide whether or not the warning applies to you. The CMs would only stop you from riding if they thought it would be a serious safety problem. I'd suggest bringing a sling, even if he has the shorter cast by the time you go. That will help support the arm (casts can get heavy) and also help keep the arm contained a little more while he is riding. Of course, also talk to the doctor about any restrictions/suggestions.
You can look at the park maps for the attractions with warnings - they are marked with a red triangle on the map and also on a sign at the entrance to the queue. For these attractions, you can warn your child ahead of time that these ones might not be good to ride. Then talk with the CM at the entrance before getting into the queue. I'm going to give a little description of the different attractions and what about causes the warning.
For
Epcot here are the attractions with warnings:
- Mission Space - both the Green (more tame version) and the Orange version do jerk you around some. Most of the motion is front to back. The Orange version has a centrifuge in addition that spins you around, simulating G forces.
- Test Track - has some sudden stops and some swerving to avoid obstacles, along with fast driving on steeply banked 'road'. Not a whole lot different that driving on the highway at highway speeds
For
Magic Kingdom:
- Splash Mountain - several drops, including one big one. The drops are straight and there are no sudden changes in direction
- Big Thunder Mountain Railroad - runaway mine train roller coaster; nothing much more to say except it is going fast on some curvy tracks. There are some fairly fast changes in direction, but since it's on a track, the changes are not that sharp.
- Tomorrowland Indy Speedway - the main concern is being hit from behind by other drivers, which causes a pretty big forward jolt
- Space Mountain - roller coaster, fairly smooth
For
Disney Studio
- Star Tours - bumpy simulated trip in outer space. Does jostle you back and forth and side to side.
- Rock N Roller Coaster - fairly smooth, but does go upside down. This is one the CMs might recommend he not ride.
- Tower of Terror - goes straight up and straight down. The number of times it goes is variable (and is a surprise on each ride). During the 'up' portion, you can feel like you are floating and the 'down' portion may jolt his cast as you get to the bottom.
For
Animal Kingdom:
- Kilimanjaro Safari - bumpy, similar to travel on rough, gravel roads. Some portions are fast.
- Kali River Rapids - this is a water ride in a raft with seats. It turns as it floats in the water. There is nothing really jarring about the ride, but you could get anything from just a little wet to soaked to the bone.
- Expedition Everest - a fast roller coaster, but fairly smooth
- Primeval Whirl - This is like combining a roller coaster with the teacups. It goes up and down, but also twirls around unexpectedly. It can be very rough and hard to keep yourself in postition, even with 2 hands to hold on with.
- Dinosaur - this is quite rough with sudden changes in direction, forward and backwards, side to side.
You could also try seeing if you can get a better idea about some of these attractions by looking at videos on youtube.com
one 'problem' is that some of them are kind of dark, so you may not be able to see much in the videos. If there are some you are not sure of after you get there, you can send one member of you party on the ride to get an idea of just how rough it is. The CMs may allow you to do sort of a 'child swap' with the rides you are testing so that you don't have to wait in line again if it will be oK.
Also, you didn't mention how tall your 8yr old is, but some attractions also have height requirements. Here's a
link to the DIS Site's height calculator.