Ok, that's not how mustard gas works, he's dead.
Mustard gas is not a breathable poison, it's a vaporized form of bleach that covers you and your clothing and rots you apart. The only reason it kills you from breathing it in, is because it rots your esophagus and you drown in your own blood.
Gas masks don't actually work on mustard gas either, at least not fully. You might not inhale it or have your eyes and nose rot off, but your body will burn and blister as your fingers and toes fall off from being soaked in bleach.