Hi there, Carla!
My dad was a double amputee, having lost both of his legs, so we learned lots about navigating the theme parks with him. My dad always looked forward to visiting
Disney's Animal Kingdom to ride
Kali River Rapids.
There are now lockers available not too far from where you will enter Kali River Rapids, and they are free for guests of the attraction to use while you are waiting in line and on the ride. There would be two things to consider about the lockers. One, depending on your husband's height, is whether or not his prosthesis will fit. The lockers are fairly generous, and I know we've squeezed two backpacks in there, or a backpack, a couple of jackets, and a handbag. The second concern is getting him through the queue to the attraction. Depending on how mobile he is without his prosthetic leg, you may not have a problem here at all.
My dad always left his legs on. This attraction has a moving platform, but for guests needing extra assistance, there is a way to board so that your ride vehicle is stationary. For balance purposes, my dad always went this route. He was able to take his time getting down into the vehicle this way.
As for getting his legs wet - we always had either a large plastic trash bag (think the kind you use in the yard) or a couple of rain ponchos. We would either slide his legs into the trash bag or wrap him up in the two ponchos, and he was good to go.
Have a wonderful time on your upcoming visit!
Anna