Hmm... Is it super cold/dry outside where you are? This might be something that will happen every winter. Dry air from your home could be causing his humidity to drop too low. Can you get a regular humidifier from walmart or something and put in his room? You can probably find one of those very cheap. Even a crappy one would do, just sit it near his tank. If the air around him isn't super dry his substrate probably wouldn't dry out so quickly. I don't have a hygrotherm, I just mist several times daily. That might be a less expensive way to fix your problem.
Again, until you find a permanent solution you like, you can always moisten his soil a little more to compensate. Maybe even make a very wet spot on the opposite side of the tank from where he's sitting and let it evaporate out.