Cas can't imagine that he didn't. He knows Dean, that he's a fighter, that he never gives up. If Dean had told him he had just accepted his fate without question that would have been a surprise.

"Eventually I found out, that you were Death, and I felt ... I'm not sure. A kind of relief, because I knew that you were not lost to me entirely. I didn't come to you right away because I couldn't, and then it went on for so long. Things have only now, after all this time, started to return to some kind of order."

Something like order, something beginning to look like order anyway. Castiel only barely just trusted heaven on it's own long enough to do this. Now that he is here he dreads going back. Looking at Dean he feels like a man who was dying of thirst who has just found water.