The toxins would probably effect the others so that is least likely. Encephalitis is more difficult to diagnose however if weeks go by and there is no improvement or he gets worse, he could have bacterial encephalitis. A round of antibiotics would help but this is last resort so I'd wait until you have exhausted all other treatments and much time has passed.
Just keep up all the love and supportive care, he knows he's being loved on. I've had birds that were sick for long periods of time with different issues and many of them did heal with time. The body of a healthy bird has amazing abilities to heal if given proper love, care and time.
Definitely keep us posted, I am keeping you both in my thoughts and prayers!