Best guess is an injury wound at this point. Whether or not it gets infected remains to be seen. Uronema, mainly seen in Chromis, is another possibility. But more spots should show up if it's Uronema.
If this were my fish, I would soak his food in vitamin supplements (Selcon, Zoecon, Vita-chem) and step up WCs. The former will give his immune system a boost; the latter will help maintain clean water conditions, reducing the possibility of infection.