Hello, I'm Dr Gary. I've been practicing veterinary medicine since 2007. I look forward to helping with your questions/ concerns.
I'm sorry to hear about your cat. It doesn't sound like the fever is resolving. That fever can lead to joint pain and the signs of discomfort that you're seeing. The cause of the fever is the question now. It could be infectious, inflammatory, auto-immune, cancer, ect.
I would want to use an oral antibiotic. The injectable Convenia is nice, but I don't think the spectrum is going to be enough here. I would go with something like Clavamox or Baytril or Veraflox or Clindamycin. These all have wider spectrum of activity. I would also use an anti-inflammatory like Onsior or Metacam
. Those can help bring the temp down, no matter what the cause is.
Once the temp comes down, he should start improving clinically. If all else fails, I would start a steroid (Prednisolone). This is in case of auto-immune or cancer as the underlying cause.
There are vets trained in chiropractics, but the issue here is not musculoskeletal, it's due to the fever.
I hope this helps, let me know if you have any other questions.