I just read the entire way through this thread, and first I want to say that I'm so very sorry that your Grey is sick and has been sick for so long without any help, and that you cannot find an Avian Vet who can even give you a proper diagnosis. That must be very frustrating and scary...
I have a few questions about how your Grey is doing now, because if I read this correctly, now he has no proper diagnosis at all? You don't know what is actually wrong with him now?
If it's not Aspergilliosis, and they are telling you it's a Gastrointestinal problem instead, why were his main symptoms sneezing, breathing issues, etc.? Is he still today having the same respiratory issues like sneezing, wheezing, other breathing issues, etc.? If so, then it has nothing to do with his Gastrointestinal tract at all..
I have to suspect that if your bird was sneezing and having symptoms of an Upper Respiratory illness for months and months, and still is, that this means that his original illness had nothing to do with his Gastrointestinal Tract at all, and it still doesn't. He may not have Aspergilliosis, but he most likely has some type of Upper Respiratory Infection...Can I ask, why does this new Vet tell you that your bird does not have Aspergilliosis? Did he do an endoscopy to collect a tissue sample?
I think the only reason your bird now has Gastrointestinal issues is due to the multiple rounds of Antibiotics and anti-Fungal medications without the assistance of any type of protective medication for his GI Tract...That is to say that your bird is and always was suffering from an Upper Respiratory or Lung illness, but these Vets gave him far too many medications without a proper diagnosis of his illness, and now he has a secondary-illness in his GI Tract due to all of these medications. So now he has multiple issues...
Also, if your bird did originally have a fungal infection in his respiratory system, such as Aspergilliosis or another fungal infection, all of the antibiotics you were told to give him most likely made the original fungal infection much, much worse...
I believe you said you are giving him Lactobacillus now? I don't know if this treatment is called "Probiotics" in Russia, but that's what we call it here, and it's a very good idea to give him Probiotics, as they will replace all of the healthy, normally-occurring bacteria throughout his GI Tract that all of those Antibiotics killed-off, possibly making the fungal infection worse, or even causing another fungal infection in his GI Tract...
Either way, if you bird is still sneezing, wheezing, having breathing issues, etc., then he's had a Respiratory illness this entire time that really needs to be properly diagnosed and treated. I suspect it's not Aspergilliosis, only because of the long amount of time he has been suffering from the respiratory issues. Aspergilliosis would have most-likely killed him by now, so I doubt that it ever was Aspergilliosis. However, if he's still having the respiratory symptoms then he still has the original illness...
I'm so sorry you're going through this...