Kit, I hate to make this suggestion (and that's ALL it is) but have you
and / or your vets considered the possibility of some kind of neurological
damage?  I am not educated or trained at all in animal medicine.  This
thought came to me as the symptoms as you've described them sound very
similar to many of the animals I see in the wildlife rehabilitation center
I volunteer for.  The vet tech there has diagnosed the animals with those
symptoms with neurological damage, most of them birds or other animals
who have flown into a window, gotten hit by a car, etc.  I could be way
off-base, but thought I'd throw in an idea.  Hope Charlie gets much better!
He's very lucky to have you caring for him and loving him as you've been
doing.
 
Queen Zen, Queen Bu and their humble servant, Laura
[Posted in FML issue 3107]