Crested gecko substrate?

If he ate the soil then chances are he'd eat anything else you use. Some people can keep theirs on eco earth or soil and be fine and some will have problems with it. If you want you can use eco earth and lay a paper towel over it and see if that aids in humidity. Personally I didn't find eco earth helpful for humidity, just messy so I use paper towl and repticarpet. I have no issues with humidity. Maybe you need a new hydrometer? Analogue ones are way off I've found. So I'm using the digital zill one, very accurate. My habba mist sprays once every 6hrs during the day and every 3hrs at night.