Please don't quote me or hold me to it but research hole in the headdisease. It looks like there are more brown spots than the one your referring to. Google pics and see if they match up. What type of betta is it? Could it be the colors changing. Is he still eating and roaming around? Not a betta expert just hoping to get more info from you so the true pros can guide you right. Good luck hes a beauty!!
Edit sorry it looks like a gourami. my bad
Sorry i'm not a fish disease pro so hopefully someone can chime in soon to give you more info about his situation. In the meantime if i were u i'de be looking up possible other diseases or remedies if it is in fact HITH.