That looks like it may just be a small normal cut. He may have gotten it from a sharp edge somewhere in the cage, so you may want to look around and fix whatever it may be because it can happen again.
I've heard before that you can put Neosporin on cuts for chameleons and its safe, so try that maybe. Clarification on this may be needed. If he's acting his normal self then you shouldn't have anything to worry about. It may even just be part of his coloring.
Im no expert so I would wait for more responses.