The problem with anything I can think of is it doesn't seem like he should be getting worse at this stage. FCE or ANNPE do not get progressively worse, the dog is stable a couple of days after it happens. Rescuedogmom's dog Stella lost her bark initially with FCE but it came back after about 2 weeks.
A dog with coonhound paralysis should show maximum symptoms in about 10 days, but it's been over 4 weeks since this started. Here is a link with the progression of coonhound paralysis, though this does say "typically", which I guess does not mean always.
Bully (quadriplegic bull mastiff with FCE) actually lost his bark and I had forgotten about that.https://vcahospitals.com/know-your-pet/coonhound-paralysis wrote:The symptoms typically progress over 4-5 days, although it can take up to 10 days for maximum symptoms to appear.
Here's another mention of Bully.karkorny in https://handicappedpet.net/helppets/viewtopic.php?f=4&t=15754&p=83854&hilit=lost+bark#p83854 wrote:I don't think he had one accident in the two months it took for him to get back on his feet. When it was all over, Bully limped and couldn't turn his head well, and also lost his deep, booming bark (sometimes a blessing for me and the neighbors).
Kobe's Australian shepherd lost his bark with FCE but it came back.
I don't know, it may still turn out to be something else. Going only on the way it started (which resembled FCE/ANNPE), one piece of standard advice for FCE is to try to keep the dog's spirits up because a small percentage of dogs will just give up due to being stressed, vet visits, a worried atmosphere in the home, etc. I think this only applies to around 15% of FCE dogs. If I remember (it's been a while since I checked and information changes) the lowest rate of recovery from FCE I had seen was 69%, and my dog's physical therapist said it was 85%. Being positive is much harder when you don't know for sure what's going on. Going back to the point made at the beginning, if he seems to be showing real physical deterioration (which your description kind of sounds like), FCE shouldn't be getting worse at this point, so it doesn't fit, either.
If he isn't drinking, sub-q fluids are easy to do at home if they give you the supplies (you may have already done them before in the past).
I am so sorry. I hope they will get to the bottom of it for all of your sakes.