"You didn't seem too interested in dragging your ass out of heaven before. What changed your mind this time?"

No, Dean hasn't figured it out. Yes, Cas is acting weird, yes, he was using weapons instead of angel mojo, but it's not the first time Cas has done that, and y'know what, it's not entirely unreasonable to think that he tapped himself out trying to hold the Leviathan's back.