Jon gives a heavy nod, which indicates his personal understanding about such an experience. He can put the pieces together of how he might end up where he was told (if it's true), but it still makes him feel sick with it. He would have given his life to ensure the Night King and his army would be defeated. He never imagined he would give up so much more.

His gaze remains on Josie though, because she's young, she's stuck in Hell, and clearly, whatever she hears about in the future has stuck painfully with her. He was given advice when this all first happened to him, and he wishes to share it with her in hopes it might help, but first-

"Can you trust what they told you to be true?"