Hi Lesa ~
Disney does a great job of trying to accommodate as many special needs as possible. You can find
information regarding attractions for Guests with Mobility Disabilities (click on link provided) including those that allow Guests to remain in their wheelchairs or ECVs, those that require Guests in ECVs to transfer to wheelchairs to board and those that require Guests to be mobile to enjoy.
Under the attractions listed as "Must Be Ambulatory", it would likely be difficult for your brother to enjoy any of those attractions as there are lots of steps and moving walkways that cannot be stopped for Guests to board.
Under the section showing "Must Transfer from Wheelchair/ECV", Splash Mountain, the Carrousel, Tomorrowland Speedway, Astro Orbiter, Space Mountain, Pirates of the Caribbean, Seven Dwarfs Mine Train, Maelstrom, Kali River Rapids, Primeval Whirl, Expedition Everest, Mission: SPACE, Test Track, Spaceship Earth, Rock 'n' Roller Coaster may be difficult because of the design of the attraction vehicles. Most you have to step down in (i.e. boats) or over a threshold to get in the attraction. For Haunted Mansion, they can stop the moving sidewalk but he would have to step up to get in the ride vehicle. For Tower of Terror, he would have to be able to step up a few inches. You could request that he be placed in the front row which would make it a little easier.
Keep in mind that you or someone in your party will have to assist him as Cast Members are not allow to physically assist Guests when loading and unloading attractions.
I am limited in space but hope the link and other information I've provided gives you the answers you need. If not, please feel free to visit again with all of your questions. It is a pleasure to assist you!
Wishing you minnie magical memories :-) Allison V.