Sorry to hear that, sounds horrific :OWell it seems whatever this issue is, it is fatal. It's also spread to almost his entire head, plus it seems like it's caused the head to cave in a bit as well.
He's on his way out as I write this. I had him for almost 3 years and it seems their lifespan is 4-6, so I'm thinking this might have something to do with age. In any case, he is/was a great fish.