My 9 year old male Tonkinese has trouble breathing and has wet sneezes often. Any ideas?

My male Tonk is 9 years old and is slightly over weight. He is 9 lbs while his 8 year old sister is fine and is 5 lbs. He would sneeze often and had blood come from his nose 5 months ago and the Vet put him on Novamoxin 250mg/5 ml 100mls a bubble gum pink liquid that I had to give him 2mL orally every 12 hours for 14 days. I kept it refrigerated and shook it well before using.

He hasn't bleed from the nose since and his sneezing went away until about 3 weeks ago. When he's sleeping he breathes quietly and will have the occasion forced air breath out. He will have fits where he has difficulty breathing and is almost water logged and ends with him sneezing a wet sneeze and spraying nose snot. I usually can have a Kleenex and wipe him clean so that he doesn't lick it up and digest it about 10% of the time but since I'm not home often I can't account for the other 90%. Does anyone know what this might be or what might be causing it. The Vet 5 months ago said that because his teeth aren't the best or the cleanest that he may have an infection in his gums and that is affecting his sinuses.

He is a very content cat and is getting up there in age so I don't want to put him through any shock or extra excitement but I don't want him to suffer with this affliction either. I have some of the bubble gum pink Novamoxin remaining in the bottle from 04-28-2012 and since it has been in the fridge this entire tire could I safely shake well and give him a few doses to help clear him out again? Or is it past it shelf life or effectiveness period.? Could it harm him more than help him?

I feel like what he needs is to visit Arizona and have the dry heat dry him out and he'd be fine but when I can't afford to go I'm sure not going to be able to pay to have him go on vacation...lol. Any Vets or Veterinary students or graduates out there that can help me out with this? Thanks