It's good that Zerxus has done breathing exercises before, because Bruce's experience with calming distraught mortals is... limited. Even doing this has thoughts (or at least emotions and under the surface memories) of Jason trying to raise to the surface.

Mostly, he leaves Zerxus to it, and rubs his back in a continued, steady, pattern.

"Better. Keep doing what you're doing." Bit of an order, at least, though the first hadn't been intended to be.

I'll give him water, take him back to bed and investigate. At least this isn't likely all about me. He also just wants to go back to bed and curl up around Zerxus.

And if Asmodeus had been concerned, Bruce would have been... shocked. And very, very alarmed.