I've seen this too. It may be contagious, so don't keep him with other snakes, and take precautions not to transmit the possible pathogen to other animals. I had a diamondback who developed this same condition, and soon after a broad-banded in the same cage developed it as well; they soon stopped feeding. A midland was also in the cage- I transferred him to another cage, and he never showed symptoms; in fact he's still going strong, 2 years later.

I don't know what it is- my vet was at a loss, but he doesn't deal much with herps. I couldn't find any info about it online, either. I ended up having to put both snakes down.