I am sorry your dog is having such problems.
My thoughts would be:
(1)a growth or tumor - sinuses, behind L eye
(2) severe bacterial infection in nose or skull
(3) severe fungal infection in nose or skull
If your dog were my patient, I would strongly encourage you to see an internal medicine specialist at this point since it has been 6 weeks and is not resolving with the treatments given so far.
The specialist may want to do xrays of the skull and nose, perhaps a scope exam of his nose and sinuses, they may even suggest and MRI or CT scan of his head. He also may need to see an opthamologist (eye specialist) to evaluate his eyes, especially since he has lost vision in the left eye.
I wish you the best of luck with your dog.
Dr. Tanis!
Either of the infections *may* respond to prednisone, because they would cause a lot of inflammation, so the pred would help reduce the swelling and such.
However, prednisone also reduces the body's ability to fight infection at the same time, so it could be possible that an infection could have actually gotten worse at the same time, so perhaps it came back with a vengance after the pred was stopped? It is just a theory. So if the underlying condition was not treated as well (let's say, with antifungals to kill a fungal infection) then it could come back, possibly worse than before.
As for a growth or tumor, I would not think it would be responsive to prednisone. However, it is often prescribed for cancer patients to help them feel better and reduce overall body inflammation.
I think you have a very atypical medical condition going on here with your dog, and I agree with your vets that it is strange that it was responsive to the pred, yet is not really responding to the antibiotics. I am most concerned about a primary brain lesion or growth in your dog, perhaps behind the left eye.
I beleive an internal medicine specialist referral is your best option to try to get to the root of the problem as quickly as possible, for the health of your dog.
Good luck!
Dog Veterinarian
10 years experience in small animal emergency medicine
You are welcome, and good luck!