Hello Johnna,
This is my first post to this group, although I am a long time lurker
and have found much great and helpful advice here for my kitties over
the past year or so!

My cat Bob (an absolutely gorgeous, handsome and sweet orange Manx
tabby) had a similar problem.  We came home from work one night last
fall and his
third eyelid was nearly covering one of his eyes.  He was taken to our
vet who was able to see a tumor inside of his sinus by looking in his
ear.  The tumor was putting pressure on a nerve that runs across the
side of the face and it turns out that he had some facial paralysis that
accompanied the eyelid condition on that side.  We took him to an
emergency vet hospital that offers more kinds of
care.  They had us get him an MRI and the growth turned out to be in a
location where it was operable and they performed surgery.  Biopsies of
the growth also found that it was benign.  Unfortunately, Bob's
condition just declined in the weeks after the surgery.  He became
lethargic and
mostly just slept.  He began to hold his head cocked at an angle and
another growth appeared on his neck that grew very quickly over a few
weeks from the size of a pea to the size walnut.  He
was obviously in pain and had stopped doing all of the things he usually
enjoyed.  It has been
nearly one year since we took him to be euthanized to end his suffering.
 It was a terrible decision to have to make but I feel certain that it
was the right decision not to put him through any further treatments or
diagnostics.  We miss him every single day.

Early symptoms-wise, there were a few signs that we were able to see
only in retrospect.  One was a bout of what we thought was allergies or
a sinus infection about 3 months prior to this incident - he was just
sneezing a lot.  He was treated with antibiotics and prednisone and it
had resolved quickly. In retrospect, it might have been the start of his
tumor but it was too small to be seen by our vet.  Also, over the weeks
preceding his illness, he had been slightly less adventurous with his
jumping (i.e. had not been jumping to his usually favored high perches
around the house and had in fact also lost his balance while jumping
down from a window ledge that he usually negotiated without difficulty.)
 Also, only noticeable in retrospect, he had been even more
affectionate than usual for a day or two before this condition became
noticeable.  I remember the night before his eyelid dropped, he had
spent the whole night sleeping nestled in my neck.

I'm sorry that I'm writing back with such a sad story and I hope that it
turns out to have no relation to what's going on with your boy.  I don't
have real advice to offer since it sounds like you are very devoted to
him and you are on top of the situation.  He is very luck to have you
looking out for him.

It could be a herpes virus. L-lysine can help that.

Wendy