Thanks, all. We had our follow-up appointment on Saturday, and his platelets are improved -- now up to 165 from 35. That's still not at the normal level, but definitely a less dangerous number and indicates that the steroids are helping. We were really happy to see an improvement. The most recent bleeding stopped sooner than last time, and his gums and ear look totally healed now, which is encouraging.
Right now he's taking the steroids and antibiotics for another couple of weeks, and we'll test again. We're not sure, though, if he'll need to be on them long-term or not. Our vet is going to speak to some people at a larger vet hospital in the city -- the main vet at our clinic is on the board there, so they can pick their brains sometimes -- and see if they have any ideas. She didn't think it was tick-related, because it's not really seen in this area, but we're keeping all possibilities in the back of our minds; I think that's one of the things she was going to ask about. I will bring up panleukopenia on our follow-up appointment. He didn't have any enlarged nodes on the last check, on the weekend, but I'll make sure that's checked for again too. Also, his fecal parasite test came back clean.
So still a mystery, but at least improving! Thanks again for the advice and suggestions, very appreciated.