I'm going to say it's not the deer your son shot. If that is an exit wound, the shear energy of the bullet should've paralyzed him, at least temporarily, by passing so close to the spine and major nerves.
Amazing sometimes the things that happen with bullets and knives. I've seen some crazy "close calls" where there's almost no explanation how something worse didn't happen... just a few mils either direction and the results would have been disastrous.
Hell, I remember a guy with about an 8" knife blade buried to the hilt in the middle of his back... literally damaged nothing major and he walked out of the hospital 2 days later - blade slid right in between his heart and aortic arch. Luck f00ker shoulda gone and played the lotto that day instead of going to the teenie-bopper house party he got jumped at. lol