An update for you all -
We took Cassius back to the vet on Saturday. The vet says we can pretty much rule out parasites or fungus at this point. Our next step would be to do skin biopsies and/or bloodwork (to rule out hyperthyroid, along with taking an overall look) but before doing that, she suggested that we try steroid injections to see if he responds. If he did, that would mean that we're pretty much dealing with an allergy. She also suggested that we try him on a completely different, specialty food with ingredients he's never had before.
So we are feeding him duck and rice pellets now, and we went ahead with the steroid injection. For the first 24 hours or so after that he was very down, obviously wasn't feeling well, wouldn't eat and wouldn't get off the heater except to go pee. But then, he perked up and is now acting all lively and happy about life. We hadn't quite realized it, I guess because the change was so gradual, but he had been really acting sluggish and depressed. Now he's back to his former self, taking swats at us when we walk by, running around in the courtyard acting like a maniac, even when he's just sitting there he seems much perkier. It wasn't until we saw him doing all this that we kind of went, "Oh yeah, he used to do all that stuff and he hasn't been." He also chased a big puffy orange tomcat out of the yard the other night - must be the steroids.
So we're very encouraged that he seems to feel so much better, even though he's still bald. I guess that means he's got some sort of allergy? The big question now, is to what, of course.
We're also keeping a close eye on him in case he starts pumping iron and decides to run for governor...