Looks to me like the entire enclosure is extremely dry. Also, I am not certain what type of substrate that is, but it is a dangerous one to amphibians. That would be the equivalent of filling your bed with toothpicks.

I suggest you increase the humidity and provide a water dish a few inches deep in addition to a daily misting with water. I also highly suggest you remove that splintery wood substrate for something more frog friendly. The final issue is temperature. You really need to have a thermometer that works if you are using external heat.
Arboreal frogs hiding on the ground is indicative of many different probable causes, but in this case I suspect the enclosure is too warm and dry for him and he is attempting to conserve water by hiding in the coolest place he can find.