What is going on here?

Sebastien, age approx 5. Been large and healthy since I got him from my
vet about 3 years ago (he was my vet's son's ferret who left him with
Dad before going off to college). We've always called him Sebastien Von
Winkle since he sleeps hard, routinely does the 'SND' trick on us. Very
laid back, mellow easy-going guy who normally plays hard and sleeps
hard.

I noticed in the past few weeks he's started losing weight, has been
going and napping during playtime and cage cleaning time. Fasted him
for about 4 hours, took him to vet Monday. BG was 85; pretty normal
after that long with no food, so we ruled out insulinoma. Vet
empirically suspects some sort of neoplasm (Lymphoma? But no enlarged
nodes at this point) as the only obvious answer without exhaustive
diagnostics (bloodwork, xrays, ultrasound) but overall he seems pretty
fine; weight loss being the only obvious symptom. He's eating,
drinking, pooping, peeing as usual. Actually his poop today looked
really nice; very normal. (Yes, we ferret folk are weird, aren't we?)
Vet noted no fever, he's has no vomiting, no anything out of the
ordinary. Put him on Pred 2x a day to give him some energy, stimulate
the appetite, and if it is some sort of cancer, that's about the
simplest treatment.

I've been using the same protocol as my insulinomic Burleigh; water
down a bit of chicken Gerbers with the Pred, and then follow with a
slurry of ground-up kibble and water to give something on the stomach
to additionally buffer the Pred and give some protein. Sebastien has
seemed to do fine on that, eats it very readily, cleans his bowls.

This has been the treatment since Monday, he's been fine, until
yesterday...now he occasionally makes a convulsive sort of movement
during his feedings, like, his whole body kinda stretches out and
retracts, and after I put him back in his cage and check in on him
seems to be swallowing a lot, licking his chops and salivating,
occasionally excessively. I first thought Megaesophagus, but it causes
regurgitation of food, and he's keeping his food down fine. He eats
his crunchies in his cage and I've not seen any obvious sign of
regurgitated food. Seems like some of the symptoms of nausea, but
no grinding teeth, pawing at mouth or chin-rubbing.

Racking my brain. Been trying to search some of those terms online but
not finding anything useful or relevant. What am I missing here? If I
take him back to the vet, what should we be looking for?

Anyone?
Thanks to all in advance!

~Sherrie aka daoine o' the ferret queen
and the weezils: Burleigh, Pandora, Ashby, Emma-wheee, Sebastien and
Dorje
and all those over the Bridge...

[Posted in FML 8189]