Oh I dunno OR - he still looks tense in the alternate photo - his shoulders are set and his head's still down...on second thoughts maybe it's the shotgun?
I've had a second decco at the first photo, and that dogs eyes are focussed more on the gun. Maybe he's gunshy.
We would break a dog in to a shotty by waiting until it was feeding, then fire of a couple of cartridges each time - it seemed to work.
I've got an old retired working dog at my hearth who is very gunshy...or whipshy. When the fires going in winter and I put on fresh wood, it gets a bit vocal, sounding much like a whip or a .222 and the old fellow doesn't hang around - he's off like the proverbial shot out of a gun.