Small beardies are sometimes stressed in big enclosure's. One my beardies was in a big enclosure when it was smaller and was stressed pretty bad. Once I moved him into a smaller enclosure he felt more secure, started eating more, etc.
How big long is your beardie? I'm not real good with the first hand knowledge of growth rate but is he smaller than about 10" snout to tail?
Another thing you can do if you don't want to move him into a different enclosure is partition the current enclosure off with some plants or other objects so it doesn't seem like he's in such a wide open space.
It's kinda like when you were a kid and when you were tucked in you felt safe and comfy.
Hope this helps!!
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Rob Talkington