I've seen this behavior before, and believe it or not, it's actually a sign of stess.
My bet is that if you put him in a smaller enclosure like a 28 qt tub with a couple of hides, give him a week to de-stress, and offer him a rat, he'd hit it like a ton of bricks.
At the very least, take a bunch of crumpled up news paper and fill up all that extra space in your 37 gallon.
I'd be willing to bet a doughnut that he's not eating because he feels exposed...I know you're probably thinking that if he feels exposed he wouldn't be laying between the glass and the hide, but rather inside of it. Truth is, BPs can be funny that way.