Cat keeps throwing up

My oldest furbaby, Furby, is going to be 11 in February. For the last
6-7 months he's been having problems that the vet can't seem to
resolve. He gets a short hacking cough, like he's got an allergy or
post-nasal drip. This triggers his gag reflex and he throws up. The doc
started him on "allergy" shots but they just don't seem to work. He's
not retaining the calories or nutrition he needs and is getting
skinnier and skinnier.

I'm really worried about him. The vet did tests and says he can't find
anything wrong. Nothing wrong with his thyroid, etc.

Now, he's always had "drippy eye". I have the same thing and it's
because of my sinuses. Could that just be getting worse and now his
sinuses are dripping inside, causing him the hacking cough that
triggers his gag reflex? Poor baby can''s seem to keep his food down
for long.

Any ideas on how can I stop it? Is there a decongestant for cats? The
vet's at a loss and I can't afford any more visits for awhille anyway.

CS