Everything you are saying seems like it is on the money, I'm at a loss for what might be up with him. It does look like his right side is sticking out a bit more, just give it a gentle push, if you can feel something lumpy then he might be a little impacted, if not then he is fine digestively. You aren't using any wood finishing compounds, hobby glue, or solvents in the house are you? Is your wife using carpet cleaners or strong cleaners in that room or any adjacent room? I'm just running through any random possibility here.

Outside of those factors, the only thing I could think of would be the water source, the building materials in your house, or even the land it's built on. If any of those are questionable, symptoms will show up in a frog long before a human.

As for humidity control, I use a reptifogger, and mist if the dirt dries a bit. Just fair warning on the reptifogger though, if you use anything other than distilled water, it will quickly scale over.