I've not heard of anyone with disabilities going on the Segway tour. I do know that some people with disabilities that affect walking have bought and use their own Segways, but WDW and DL don't allow their Segways to be used at the parks. (Unless they have changed from stuff I read in January).
You could try calling the numbers that WDW lists on their page of Disabilities information:
Customer Service 1- (407) 939-6244
Guests with Disabilities 1- (407) 939-7807
or TTY 1- (407) 939-7670
I'm not sure how good an idea a surpise might be. It's possible that they might not be able to say for sure whether or not he can participate until he is there. It would not be a good surprise if he gets turned down right after finding out he is going.