Nice cage...but I think I see your problem.

You've got this great cage that allows him all of this room to move and stretch out AND...be seen.

Like one of the previous posters said...cover the sides to "cramp him in". He's used to a smaller area (for him small = secure). Give him more cover and hiding spaces.

If you prefer to be able to see him instead of having to search for him everytime you want to show him off or do something with his cage you can slowly remove some of his cover one piece at a time (be sure to leave any items that appear to be his "favorite" for last).

Since you have a wooden cage a simple way to cover the sides and back would be to either purchase some of that decorative aquarium background or buy a few yards of dark cloth and keep it in place with tacks. Over the course of the next few weeks (or months...depending on his ability to acclimate) you can lower the cloth or background by a few inches at a time. This will give him time to slowly get used to being "on display".

About 15 years ago the first lesson was taught about keeping reptiles and amphibians was "A hidden herp is a happy herp."