I think that more than anything at this moment, his biggest fear is to learn that Sam ended up in hell. He knows what hell is like, and he can’t bear the thought that he is in heaven, while Sam might be in hell, alone.
But imagine Sam walks into Dean’s bedroom one day and notices that there are *two* pillows on the bed instead of just one. He frowns a little, then his eyes widen. He runs to check on Castiel’s room and sure enough, Castiel’s pillow is missing. And that’s how he finds out that Dean and Cas are together.
Dean and Cas find out he knows when they find Dean’s bed strewn with rose petals and a note on the pillows that says “Finally!”
[image: Sam Wilson and Steve Rogers stand side-by-side. Steve is wearing a blue sweater with a Star of David design, and he’s smiling at Sam. Sam is leaning into Steve and smiling up at him, wearing a blue sweater with a picture of a menorah that says “Get Lit.”]