If it isn't hole in the head, it could be his body rotting. I had a female who had a huge hole where her dorsal fin should have been. It took weeks for it to heal, and even then she died from another disease!
But, if you have the heavy meds like Kanaplex, or maybe even try Maracyn and Maracyn 2 combo... get him. Always worth a shot. I got a necrosis/columnaris betta, knowing fully he would not survive and true enough poor Blue died 5 hours later because the diseases were not caught before they happened!