Don’t mirror his anxiety. Just be there with him, a steady presence in the trenches. He knows you’re there if he wants to talk. A warm touch can do a lot - even a hand on his shoulder, his back.
It’s normal to feel anxious and afraid, and he’s in survival mode now. It’s only been 24 hours, and it’s not unhealthy to be holding in his feelings, at this point — there’s no right or wrong and everyone copes with crises like this in different ways. Try not to project what you think he “should” be doing right now (even though it’s well-intentioned).
Just be there - it’s really enough. Help him decipher the medical jargon if he needs, and take care of him in other ways (bringing him food/drink, taking care of kids do he doesn’t have to worry and can just focus on his mom, making phone calls, etc.)