I had moved the water bowl but maybe that will help as well, so I'm going to move it back to where I had it before and see what that does for me.
The big issue here is that he's not eating, but he was a ready feeder before the tank change. Healthwise, he looks just fine. He seems to like the tank well enough (after a few days of adjustment). When I handle him he's the same as he's always been: a little shy and nervous, but docile and settles down with me fairly quickly.
While considering other possibilities - he might just be fasting, as some snakes do - I've realized that the humidity could be the culprit. I'm going to offer another mouse tomorrow now I've taken a step in the right direction.