Was there a rodent involved anywhere? Did he miss a feeding strike? Or was he fed recently? Maybe he caught the scent of a rodent on his own scales and instinctively bit?

He should figure out fairly quickly that he's not edible and let go. Check him for any punctures afterwards. If he broke the skin, keep it clean and put some antibiotic ointment on it. (the kind without any pain reliever in it)