When I first got my blackthroat monitor as a very small hatchling, I set him up in a 29 tall glass aquarium with a screen lid and heat lamp. I placed a large, tall branch in a position where he could climb it to get closer to the heat source. The screen lid was partially covered with large books just to weigh down the lid, otherwise crickets would escape. I used moist bedding which probably helped with the humidity, provided a large water bowl, and fed him regularly. The set up has worked well for him, but I could have just gotten lucky. He was pretty small when I got him but not as underfed as yours. Yours probably has a higher risk of becoming dehydrated. Another thing you may want to do is to soak your guy daily to make sure he stays hydrated. It's good that he is showing interest in the crickets. Hopefully you can get him back on track with no problems.