There is one accessible buggy at Haunted Mansion at WDW.
It is a regular car, except that it has a hinged door. The seat is approximately the same height as a regular wheelchair seat, so depending on your brother's upper body strength, or with your assistance, he should be able to make a sliding transfer. Identify yourselves to the CM at the entrance and you will be taken around to the exit side where they can stop the entire ride while your brother transfers.
You can also ask to go into the 'stretching room' though you won't be able to continue into the ride load area. You will have to go back out, then go around to the ride exit.
Andrew