"Just like the other surgeons, the Kole tang has a “scalpel” (a sharp spine) at the base of its tailfin which is used for protection and to establish dominance. When the spine is not in use, the Kole tang will keep it folded down inside a groove. "
Looks like he was fighting with someone or maybe was caught in a net? Is he new? Are you sure it's a cut not just the spine being erected?
What are your water parameters? Good water and good food is your best bet, it should heal on it's own (if it is indeed a wound) as long as it doesn't get infected. It wouldn't hurt to soak his food in fresh garlic for an hour or so before you give it to him to boost his immune system.
If he is erecting his spine try to observe who he has a problem with in the tank.
Can you see where this tang hides his spines?
none of these are my pics by the way but I hope it helps.
The left side does look a little damaged under that scalpel.