They're developing that quickly? Sounds like some kind of skin parasites. Not just your run-of-the-mill fleas either. Most rashes and fungal infections show up as red, inflamed and weepy. I just don't know.
How is he behaving? Still active and alert? His appetite OK? I doubt this is something you can treat on your own. Is there a ferret group in your area? Perhaps they've seen this before and can advise you. I deal with mink and weasels but I've never seen what you're describing.
Wow...that sounds pretty disturbing!
Try posting a question in the Ask a Vet expert section here at MedHelp. You can post in either the Pet Skin Problems forum or the General Health forum.