This is going to be extremely long winded, but I want to share all the facts before I get some opinions!
 
We have a five year old male named Grundy. We adopted him and his cagemate from a rescue in Pittsburg three years ago. He has always been a very healthy, fat (he was almost 5lbs when we adopted him, he's now down to a trim 3.1lb), and happy ferret. Recently he has had some health issues and our vet diagnosed him with insulinoma, but I'm really not sure about the diagnosis.
 
About two months ago I found Grundy in his cage unresponsive and running a fever of 106.5. I took him to the vet where he stayed all weekend. She gave him antibiotics, sub-q fluids, and got his fever down. While he was there she said he had explosive diarrhea and she took an xray of his abdomen and said his spleen was grossly enlarged. But after the bout of diarrhea he perked right up, his fever resolved, his spleen returned to almost normal and we brought him home and finished out his round of antibiotics. She took his glucose at that time and she didn't tell me what it was but she said it was normal. She wasn't really sure what the issue was, but it wasn't anything contagious because our other two ferrets never had any issues.
 
Four weeks ago I found him again in his cage, lethargic but not completely unresponsive and not running a fever. My husband took him to the vet that day and she took his glucose and it was 53. Of course she immediately said it was insulinoma and put him on pred and sent him home.
 
Here's where it gets weird. Since the day of his appointment four weeks ago, he will not eat his hard kibble. He acts like he wants to. He'll stick his nose in the bowl and nudge the food around, lick a few pieces, but he won't eat it. Every now and then I can coax him to eat a piece but he struggles with it. We give him a bacon Bandit treat about twice a week (bad for them, I know, but he loooooves them) and he won't even eat that now. He will take it in his mouth, walk a few feet, drop it on the ground and then leave it. He's been eating nothing but duk soup, sometimes with some Hills a/d mixed in, but nothing else. We feed them Zupreem which is what he's been on for three years so I thought maybe he just lost his taste for it, even though that would be strange, and bought a bag of Evo. He won't eat that either.
 
He's been back to the vet twice and both times we've brought up the issue of him not eating his kibble and she kind of brushes it aside and says "No, it's not his teeth, he's fine." So we took him to another vet an hour away that is an exotic specialist for a second opinion. She agreed that she thought he had insulinoma because of his glucose of 53, but as she put it he doesn't look or act like a sick ferret. He's doing very very well except for not eating his kibble. We talked to her about that too and she looked at his teeth and said he has some plaque and gingivitis, but nothing more than what would be expected in a ferret his age. We told her we really think he is having some type of tooth/mouth pain and she also brushed it off and said she didn't see anything wrong.
 
So here we are. My husband and I really think that he was having tooth pain and hadn't eaten in a day or longer and that's why his glucose was so low. But maybe he really does have insulinoma as well as tooth pain. We're just stuck, and our vets don't seem to want to address his mouth and he already has diarrhea from eating nothing but duk soup and is now on metronidazole to clear that up.
 
Does anyone have any other advice that we could try? Would not eating for a day or two cause a ferrets glucose to drop like a human's would?