That's really odd...Did he just shed recently before these patches showed up? I'm assuming you've tried to see if they are retained shed by trying to brush them off with a soft toothbrush in a warm
bath? If not then I would try that, but otherwise I think I'd be getting a Skin-Scraping test done and sent out for testing by my Reptile vet to make sure this isn't a fungal infection of some kind. If it was yellow or brown in color I'd be sure it's just retained shed, but the fact that it's black/grey/purple is very odd.
Does it look like it's something on top of his skin, or does it look like his skin has simply turned a different color in those spots?