Welcome to disABILITIES.
Glad to see you made it. Sorry to hear about your DS's problem with his hip.
We r not staying on site, bit too much for us to afford, but r near by and r driving there so will have a card and his handicap placard.
You will be able to park in the handicapped parking area with your handicapped placard. Just show it to the parking attendents each time you see one. The regular parking areas have trams to take you from the parking area to the park entrance (or, in the case of MK, the place where you catch a monorail or boat to the park entrance). Handicapped parking at MK is a bit of a hike from the parking area, but the others are not bad (just so you are prepared).
If your DS is able to step up to get on the tram and his wheelchair is foldable and not to heavy to hoist up on the tram, you could park in the regular lot and ride the tram at MK.
We will bring his wheelchair with us but I really don't want him bound to it and I am not sure what rides there are that he can go on and not get his lower body all slammed around.
The rougher rides are marked on the park maps with a red triangle icon. They are also marked on a sign as you enter the line and before boarding. We also noticed something new on our recent trip. The signs at the line entrance give a little ride description (Like "turbulent thrill ride"). The link that I put in your post on the Theme Parks Board had information about Guidebooks for Guests with Disabilities. Here's a list for each park (with a few extra notes):
MK:
Space Mountain
Tomorrowland Indy Speedway. These are mini race cars that are guided by a track rail (you steer them yourself within the limits set by the rail). This isn't rough or bumpy
except if the person behind you bumps into you (which happens fairly often). The jolt that you get from being hit into might be painful for your son.
Big Thunder Mountain
Splash Mountain does have a caution, but it's not bumpy. There are 2 or 3 small drops and one giant drop (you can watch cars come down that one from Frontierland). The ride cars are tight to get in and involve a big step down and over the side of the car fro getting in and out.
Goofy's Barstormer doesn't have a caution sign, but it is a roller coaster (fairly tame one, but still a roller coaster). Becasue it's made for small children, the ride cars are kind of tight. You might want to just look at the cars going by a bit before deciding if your DS will be able to ride.
Epcot
Body Wars is a very rough simulator ride. You are sitting in theater seats, but your whole theater is a simulator, making sudden changes of direction and moves up and down.
Mission Space. I haven't been on this, but DH was and said it was smooth. The reason for the caution is probably the G forces that your body is under.
Test Track doesn't really throw you around a lot more than a ride on the highway with several fast stops. You might want to send some of your party on to expereince it and then decide whether your DS can go on or not.
AK
Kilimanjaro Safari might be OK. It's a safari ride on rough roads in a simulated African savannah. The roads have simulated ruts and bumpy wooden bridges. Most of the ride is fairly low speed, so it's like going over speed bumps in a parking lot. At one point in the ride, you go on a faster speed "chase" and it can get bumpy. That's the point where my DD (who has cerebral palsy and rides this in her wheelchair) sometimes has flying legs and arms.
Kali River Rapids isn't very bumpy. You do go down river rapids, but since you are floating on water, it's not that jarring. I do remember one spot where the boat gets dropped into the water after a "hill", but it's not that bad.
Dinosaur is very rough. You are in a small Jeep-like ride vehicle that is a simulator. It makes sudden stops, sudden changes of direction, sudden drops , and sudden starts again from a stop. Very rough.
Primeval Whirl is kind of a combination of a roller caoster and the teacups. I do NOT go on it (can't take spinning), but my DH and 2 DD's did. My youngest DD (with CP) almost fell out because she was not able to brace herself with her feet and was being slammed from one side of the car to the other. My older DD didn't get slammed around, but said her legs were tired at the end from bracing herself.
Studio
Star Tours is very similar to Body Wars, but with a Star Wars theme.
Rock N Roller Coaster. I have not been on this personally, but from what I have read and DH has told me, it's a very smooth roller coaster.
Tower of Terror brings you up and down a number of times. While you are going down, you are momentarily weightless. As you go down farther, gravity kicks in and you might be dropped back onto the seat with a bit of a jolt. When I went on, they had lap bars that went across a whole row, so smaller people had more room between them and the bar. I've heard that they switched to individual lap belts. I don't know how that impacts the drop.