I don't see where this would be any sort of a problem.
I do know that several years ago when I had one of my kidneys removed (cancer) the surgeon told me that for the next six months if I was in parks any ride with warnings for heart conditions, pregnancy or bad backs applied to me.
My daughter has a kidney problem as well and had surgery on her left kidney. Her function is not nearly as bad though. When she was just a baby I asked if there was anything she could not do and he said no. He just said always make sure she wears a seat belt.
Since it sounds like your grandsons issue is a bit different than my daughters I would just call his urologist to be on the safe side.
I agree.
The rides with warnings are pretty well marked and even if he doesn't go on those, there are still a lot of things to do and see.
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 Epcot here are the attractions with warnings:
Goofy's Barnstormer (this doesn't have a triangle warning sign on the map, but does say that pregnant women should not ride, so it is kind of turbulent)
Some of them - like Rock N Roller Coaster are pretty smooth. Others - like Kilimanjaro Safari are bumpy like a drive on a bumpy road. Others - like Dinosaur, Primeval Whirl and Star Tours are very, very turbulent and really can throw you around.
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