Sounds like you may have him on one of the "spurs" on top of the vertebrae. If so, it probably won't kill him unless it gets infected. There aren't any major blood vessels there, just muscle which would explain blood where he fell and why it quit after a little bit.

Several years ago I hit a pronghorn doe like that with a rifle. She went straight down and then got up running flat out. She disappeared into some gullies and I couldn't find her. The next day I was doing some glassing and saw an antelope with dried blood on here side. I stalked close and dropped her and it was the same antelope with a fresh bullet wound in the back strap.

When I boned her out one of the "spurs" was shattered. Other than the blood, there was no outward sign that she had been wounded. She moved like she wasn't hurt at all and was actually feeding when I found her the second time. Game animals are tough and will often survive a high shot that misses the vitals.