Roller coasters do not have any kind of disorienting effect on me, however, I was riding with a good friend of mine and he said the part going backwards through the dark really did a number on him. I could see how that could happen - that part is pretty intense.
Another thing to think about - is how your 6 year old handles big, loud, scary monsters. Because that's basically what the Yeti is and the part where he swoops down and tries to claw you could probably be pretty intense for a 6 year old.
Every kid is different. Some would probably LOVE the thrills. Some, like my DD5 would be absolutely terrified. My DD5 is terrified with BTMR, which sucks because I love the thrill rides... She also was terrified with Dinosaur, which also has loud, scary, realistic looking dinosaurs, which has a tendancy to scare not only children, but some adults as well...
I personally would start him off with BTMR and then Space Mountain. If he really loves those two, then he probably would be alright with EE...
As far as "thrill factor" goes, I would rate the Disney coasters in the following order:
1.Goofy's Barnstormer
2.Big Thunder Mountain Railroad
3.Space Mountain
4.Expedition Everest
5.Aerosmith's Rockin' Roller Coaster