The more you tell me the more I think he's still probably too HOT & needing relief, hence the digging under substrate or laying by water bowl.
How are you reading the temperatures in the cage? Petco is probably NOT your best bet for a thermostat...and it's definitely not the best bet for a simple rheostat (lamp
dimmer)...for that, just go to your local hardware/home improvement store...they'll have them, just ask. You don't want one to install yourself in the existing cord, it's far
easier to buy one that's already installed in an extension cord, ready to plug your CHE cord into. Thermostats- decent ones- cost more & you choose the temperature,
whereas a lamp dimmer is just a rotary dial* that you'll have to figure out by trial & error where to turn it to, to get the temps. you want. Petco will likely have their own
version of a low-quality rheostat control for more than you'll pay in a hardware store.(*the rheostats that I use, I use a permanent marker on the dial for "hi, med, low")
Is the CHE your ONLY heat source? Overhead heat works but it's not the best way to heat the cage of a terrestrial snake, as you waste energy: heat rises, so much of the
heat from a CHE (or a light) simply never makes it into the cage and down to where the snake is on the bottom. That's why I mostly use over-head heat in the winter, and only to add to the UTH heat from underneath when the ambient temperatures in my house are lower.
You might take a look at Reptile Basics or other online sources before you shop locally.
https://www.reptilebasics.com/heating https://www.reptilebasics.com/thermostats