Caption suggests that the subject is deceased. In portraiture of the early 1860s it was not uncommon for photographers to use clamps to steady the subject for long exposures, particularly if the light wasn't so good. Holding up an embalmed body would involve much more than the clamp indicated I would think.
He's dead now, but I doubt if he was dead at the time the image was taken. Would they have had such a clean, well fitted fancy uniform to photograph a cadaver in? He's wearing a frock coat with trim. If he was in battle, he would have had something different (and it would be dirtier).
The image under discussion in that thread is shown on page 1, but I linked to page 2 because there's information about the unusual position of the non-gun hand in post #16, and also a more obvious post-mortem of a soldier in uniform--eyes closed, arms folded--toward the bottom.
Edited to add: I think the California soldier in this thread is alive.
I would be interested to see anyone pose a corpse, standing, upright, and make it look like it's living and conscious. Ain't seen one yet, but I'm open to being convinced.