Do you think he could benefit from using a GAC?
It's hard to say. What do you think his needs may be, beyond what the scooter will take care of? He may be a bit slow getting in and out of ride vehicles, but you don't need a GAC for that - just tell the CM at the load/unload area. (The Peoplemover and Peter Pan are the only moving walkways that can't be slowed.) Can he walk up or down a few steps if he needs to?
The scooter can go in most lines, but there are some, like Pirates of the Caribbean, where he'll have to transfer into a manual wheelchair.
My advice is to read up on the attractions as much as you've got time for, make note of anything he might need help with, then go ti City Hall when you're there and explain it to them and see what they recommend. The DisAbilities FAQ will also have lots of info on different rides.
He has significant pain, fatigue, and nausea.
I've read that Star Tours, Dinosaur, Mad Tea Party, and Soarin' have been known to give nausea-prone folks motion sickness, FWIW. When I was going through chemo, certain smells triggered nausea, but I'm not sure if there's much you can do about that. I've heard you can request special allergy cleanings for hotel rooms, but I'm not sure who to call about it. I've asked for unscented soaps and shampoos, but have been told no and always end up bringing my own.
Also, has anyone had to obtain medical care while at Disney? My biggest concern is my husband sometimes develops ascites and needs to have paracentisis to have the fluid drained. At home, he can just go outpatient to the hospital and have it done in about 30minutes. Any tips on how such a procedure could be coordinated while at Disney? Especially if we won't have a car.
I've never needed anything that specialized, so take this with a giant grain of salt, but I was happy with the general ER care I got at Florida Celebrations Hospital for a broken bone. Your husband's oncologist may be able to recommend someplace better suited. I went in an ambulance, but I'm sure OKW would arrange a taxi for you.