Oh, geez! And you said he was doing sooooo good! Well, I don't know of any fatty liver specialist... have you asked your vet? I tell you one thing though, I did not like the compounded medicine your vet gave him. It might be the best in the world but, personally, I like to know EXACTLY what, how much, what quality, etc and this did not fit my requirements in many ways. First of all, alternative medicine works but you need good quality for it and because the industry is completely unregulated, you get bottles that say they contain this or the other and, in reality, they don't (this has been documented extensively -
http://www.zmescience.com/medicine/herb ... led-herbs/ -
http://www.salon.com/2015/02/03/walmart ... pplements/). That's why you need the sources that process the plant material adequately and are independently verified as to content and strength. We don't even know what was in the compound, much less if it was good quality or in the right amount. Secondly, putting supplements together is not a matter of mixing a batch of something, you need to know what is the best medium and what goes with what. This compound appeared to have SamE mixed in it but all I've read about it says that you can't crush the tablets or mix it with liquids because it loses efficacy (all the SamE I've seen out there is enteric-coated tablets). Third, if the extracts were alcoholic, they would actually make things worse. I don't know if the doctor is aware of this and compounds liver aids separately or just makes a big batch for every liver damaged animal he treats because dogs and cats can take an extract made with alcohol and it will not damage anything as long as it's in a very low dosage but not birds. Now, the thing is that the active ingredient in milk thistle needs to be extracted through alcohol so, for birds, you need to get the one that has an extra second stage where they take all the alcohol out of it and that's more expensive. Fourth, stress is real bad for sick animals and taking into consideration that captivity is stressful to undomesticated species, when it comes to a sick parrot, you need to avoid it like the plague. But medicating a bird in his beak twice a day is going to be stressful no matter how careful, slow and kind you are so I always prefer medicating in the water when it's possible and, thankfully, when it comes to liver issues, it is.
Now, if you feed nutriberries and avicakes, you are feeding seeds so maybe that's what everybody meant when they told you that this was because of the seeds. Seeds are fine but you need to reduce the protein drastically and you can't do that with a product that doesn't even give you an exact protein level (avicakes, nutriberries, pellets).
Personally, I would not do the biopsy (way too much stress for such a sick bird) and as long as he is not suffering (when you see neurological damage, you know he is at the point of no return), I would keep on trying with diet and meds but you are going to have to steep up the care 100% and change the meds and diet because, obviously, the treatment the vet gave him did not work at all and seems to have made things worse, doesn't it?