Why is he in a cabinet? *blinks*
Like LGray said, he should have access to day/night cycles. I'm lucky enough to have a lot of good ambient light in my bdrm where I keep my snake, so his tank sits out on my desk and I don't use lights to provide the cycle. But if you don't have a lot of ambient light, he needs a day/night cycle so he doesn't get confused. It's just like a person.
You said you have a 20gal. Is there a lot of foliage in the tank?
Here is my tank set up:
I put in the fake vines to take up a lot of the open space in the middle, because the two hides weren't enough.
If you've got plenty of foliage + two hides (one on the cool and one on the warm side), then it's a waiting game for your snake to settle. It varies from snake to snake.