He takes the positive feedback as a sign that perhaps Crowley could take more, though how well he's evaluating the situation might not be as significant as how much he finds himself wanting to thrust into Crowley's mouth. It's almost a relief when he starts, holding Crowley in place with both hands now as he rocks up into that heat that's oddly intoxicating.

It's only almost a relief because Castiel's starting to feel so full of pleasure that it's becoming too much; he can't possibly contain it, just like he's unable to keep quiet, his head thrown back and mouth open as he moans. While it's wonderful, he doesn't know how long he can stay in this state before it'll tear him apart --

Oh. Suddenly Castiel finds himself coming, cock pulsing in Crowley's mouth as Castiel's hips are frozen in midthrust. This is what an orgasm feels like.

He relaxes back onto the bed, breathing hard and feeling incredibly dazed.