In the photo it doesn't look bald, it just looks like the fur is black, so maybe it is a place where he is starting to moult and new fur is coming in. If you look closely does it seem like little black hairs are growing?- if it does I would say he's just staring to moult. But if you see bald skin that looks flakey or red and inflamed it could be ring worm, or fur mites. But from that photo I can't quite tell.
