I have only 1 cave, and I think it's a perfect size for him. But I ordered another one, and it should be here tomorrow.
I would've never guessed that moving the hides around can encourage them to use it! I was thinking that maybe if I kept everything in the cage looking the same as the old cage, that he might feel more safe and comfortable. The cave is over the heat pad, like it was before. I'll probably move it to a different spot later today. Then when I get the new cave, I'll put one on the cool side and another over the heat.
I kinda forced him out just now, and I gave him a hopper to eat. That was his very first hopper, since he was only eating the smaller mice beforehand. I'm glad he ate, because he was looking kinda skinny. His spine was pretty prominent. I'm guessing he hasn't been drinking enough water, either. Hopefully he'll return to "normal" in a short while.