If he had a breath he'd hold it for that stretch of time where Jack's just staring, because he really doesn't want this to break out into a fight but he also can't just sit idly by and let people be dumb about shit. They need to know what they're doing, or they'll keep doing it and maybe he's wasting his time here but what else can he do? Can they trust each other if they can't speak openly about this?
Luckily Jack seems to, if not get it then at least accept it.
"I don't think he's got it in him, no. I'll know more after tonight."
So sit tight, he's got this, you got nothing to worry about. Keep an eye on Ianto and let him handle the rest for once.