He does relax even with the reminder of their difference; here he is consorting with a demon in the basest way, and yet he finds his reassurance comforting. It's difficult to say if he trusts Crowley; he trusts him to do what is best for him in any given situation, which lends him a kind of predictability. Here, he believes Crowley's enjoying this as much as Castiel is.

"Oh." It starts out a word and turns into a hushed and very small moan. He glances down, watching Crowley at work, and he swallows audibly.