It's interesting how he still has color on his head and along his neck, then it just fades away.
Not much to think about it other than what's already been said. It's not an unknown phenomenon, but it's rare. I'd be really interested to know what kind of environment he's kept in, and if there were any significant changes or events within that environment since his last shed. Or changes to his diet...even if it's just a chance of source, if not type.
I also think it would be interesting if you kept records of all his sheds, including pictures, until he returns to normal again.
Such records might make it possible to eventually figure out what causes this.