Chronic Coughing/Sneezing

I am writing this on behalf of my mom. The cat in question is technically her cat (I live across the country), but I am incredibly fond of him and I really want to see if I can help.

My mom has a 10 year old male pure-bred Tonkinese cat. Ever since he was 3 or 4 years old, he has coughed and sneezed and sounded congested. At first they gave him antibiotics, but those didn't help. I think they thought it was asthma for a while, so they put him on oral steroids. The steroids helped for a while, but he's been on them for at least 5 or 6 years, and my parents started to notice that he would sound worse after each dose instead of better. They are slowly weaning him off the steroids, and I agree he sounds better than he did when I was visiting at Christmas, but he still sounds pretty bad.

Here is what we know it is NOT:
Congestive heart failure
Any kind of bronchitis or pneumonia (it sounds to me like lung congestion, but the vet always says his lungs sound clear, so it must be his sinuses?)
FIV or FIP (he's been tested and he is a 100% indoor cat)

He sleeps a lot, but no more than the other two cats they have, and he never seems lethargic or sick when he is awake. He snores when he is asleep and coughs and sneezes when he is awake. It's probably not relevant, but he also has bad teeth and eats too fast so he often throws up his breakfast.

He goes to the vet regularly, but they haven't come up with any new ideas of what might be going on, so I thought I would see if a large community of cat lovers has any experience with this. Thanks!!

Not to scare you, but has the cat been tested for heartworm? I understand that sometimes the symptoms can mimic asthma, and that in the past vets didn't always look for heartworm in cats - it sounds like it's only recently gained attention in the profession.

I don't know if he has or not. I will ask her. If I had to guess, I don't think it would be that because he is a completely indoor cat and this has been going on for at least 7 years if not more. He also had an x-ray followed by an ultrasound about 5 years ago when the vet thought he saw an enlarged heart (turns out it wasn't).

Thank you for the suggestion though. I will ask my mom.

He could be a chronic URI cat... Although I am not sure about the cough... Is he on L-Lysine? If not, it can help him a lot - especially if he has chronic URI due to Herpes virus.
This is the one I use: http://www.iherb.com/Now-Foods-L-Lys...454-g/653?at=0
I mix into their water and wet food...

Feline herpes might be the underlying problem. He can have the Feline PCR URD test to determine if he has FHV.

Thanks for the suggestions. I will ask her whether the vet has mentioned feline herpes, and about the L-Lysine.

Do they smoke? Rocko has respitory issues and my vet said it was probably from him living with a smoker (it damages their lungs) If they don't smoke it is more than likely the feline herpes (Fiona has it and its chronic) lysine works really well to keep her healthy though and keep from getting infections.