The sudden intensity of the squeeze on his hand makes a wince flick across his face, but that's secondary to the new information he's getting. Or inferring, really, but Jon wouldn't be so openly horrified unless...

Martin's eyes widen. "...what? Jon, they're--?"

Oh God. Oh no. "N-no, no no no Jon, they can't, I-- we'll think of something, I'm not going to let you just--"

But his free hand comes up to rub his mouth, like he can't quite choke out the word either. But his voice is firm, even if it's trembling slightly, when he says, "I'm not going to let them, Jon."