Ornate monitor enclosure question

[Grendel]

I have a young ornate monitor that I got 6 weeks ago (15inch snout to end of tail). How much substrate ( depth ) do I need for one that small. Right now I'm using just plain dirt, but I did not fill the entire bottom of the enclosure. I just put in a plastic container that is about 6 inches deep with the dirt in it. He does to seem to be interested in it, I don't see any signs of digging, and he prefers to hang out up high on the branches. This enclosure is small (28in Wx28in D x 40 in H acrylic cage), but I have a 6ft x4ftx 4 ft wooden enclosure I was planning to put him in when he got a little bigger. Do you think I should just set that up and let him loose in there, or wait?