Yeah, get that guy some tight fitting hides (plastic bowls with an entrance cut out and sanded work well). For now, cover the backside of that log so no one can see in the back (I just tape construction paper on the outside of the tank), and maybe stuff it with paper towel or moss to make it fit. Identical hides as soon as you can is important too, so he doesn't have to choose between security and thermoregulating.
The other thing you can do is, until you get plants, just loosely crumple some newspaper and fill in the space so he's not so exposed.
Check your temps (and get a digital temp gun or thermometer if you can, those dial things are trash) and a thermostat on the heatpad if you're using one. Once you've got him at 80 cool side, 90 warm, and he's secure you should be gold.