I'm so sorry to hear about your little guy... I can't really see anything in the picture, which is good-- it means it's not too bad. I would QT him right away and put some aquarium salt and stress coat in to promote healing. If this IS a disease then you NEED TO TREAT IT, PRONTO! HITH (Hole in the Head Disease) creates irrevesible damage. The hole will keep getting bigger (it's bacterial, I think) until you medicate it. Your fish will live a normal life if it recovers, but there will always be a hole. You should look it up, I've never had personal experience with it.