If it were mine, we would kill it. We would behead it with an axe or hatchett, or knife or snips if the latter wasn't available.
When I was younger and didn't realize how vicious chickens were, I had this happen to two of mine. The didn't make it, even though I tried. After they died from their wounds, I felt bad I didn't just put them out of their misery in the first place.
However, people have successfully saved scalped birds. It takes quite a bit of work and time, but it's not a sure success.
You can ensure relief from pain through death, but not always through healing. It's up to you what you want to do.
By the way, I am pretty sure another chicken did this to him. That looks like a classic scalping from a flock.
-Kim