The vet wasn't able to say for sure what's causing Bear's lower body weakness (his hind legs don't have as much range of movement and control, and his tail is mostly limp). Apparently, he might have hurt his back. He might also have degenerative osteoarthritis. That second one scares me, since it means he's only going to get worse. (I love Rat Health Guide, by the way - it's been very helpful over the years.)
The vet has put him on steroids, and today was his third day getting them. So far, I'm not sure if he's improved, but the vet said it might take as long as a week to see results. I'm also a bit worried about his tumor, which seems to have gotten a bit larger. I had originally planned to take him in for surgery if it looked like the tumor was going to start growing, but with his other problems now I'm not so sure. I'm hoping the universe will be nice to Bear and just leave him alone for a while.
I'm working on creating a single-story cage for him. It's basically just a plastic storage container with some aluminum screen material on top - I doubt he's athletic enough anymore to make it out of the container, even if I chose not to give it a cover, but I figure it's better safe than sorry. I'll probably put him in the new cage tonight, so I can see how he deals with it before work on Sunday.