It looks like he could have scraped it off? Similar thing happened to my guy a while back; he chipped the front of his casque from trying to wedge himself between vines. Vet prescribed chlorhexidine and silvadene cream but definitely bring him in to get a professional opinion.
I just realized, mine was around the same age when this happened.. If you've never had this issue before (7 months with no incident, and then this happens right around the time he is maturing), he could be trying to tell you that it's time for a bigger cage.