It's my opinion he should have a rhinoscopy (take a professional look up his nose, basically) I know he doesn't spend much time outdoors, but could he have possibly come into contact with grass seeds, or even "foxtails"?
If there are ANY foxtails or anything similar, growing nearby your house or anywhere he goes for walks, you have to consider that as a possible irritant. And they are nasty.
Even if no foreign object is found, there is also a possibility of nasal polyps.
When something's wrong we are all inclined for some reason to think "allergies" nowadays, and you're right -allergy testing doesn't come cheap. I'm not saying don't do it. But there could be a straightforward irritant or blockage up his nose which needs fixing quickly.
However, considering possble allergens....dust is by no means the main irritant for dogs. More suspect, would be household cleaning products, air fresheners, detergents, bleaches, bathroom cleaners, furniture polishes, even scented oils etc. Even something from the vacuum cleaner or tumble dryer. Our modern houses contain many things which can upset dogs badly.
With dog in mind I only use two household cleaning products....laundry detergent, and liquid dish detergent (which is rinsed off obsessively). And if I wash floors down, or that kind of thing, I rinse, rinse, rinse with fresh water before I will even let her into the room. For many jobs I use plain hot water with nothing added.
You say he's been drinking far more water than normal recently? In the absence of really hot weather, or hard exercise, (when you can expect a dog to drink more) that can mean there is something going on, and it CAN point to problems like diabetes, or Kidney insufficency. I would check that symptom out with your vet, and even get a blood panel run, for peace of mind.
Ginger has given you some excellent advice. Veterinary treatment doesn't come cheap, but it needs to be done to make sure your dog isn't suffering from something that can easily be treated by simply removing something from your home or stopping the usage of a substance that is irritating to him. It would be a waste of money to buy the air purifier since dust is, in all likelihood, NOT the problem. The only way to find out what it is for sure would be to do the allergy testing.
The increased water consumption may or may not be related to his sneezing. At 8 years of age, he is considered a senior dog now and this is the most common age for all manner of health problems to start. Diabetes, kidney failure and Cushing's Disease are only three of the things that can cause increased water consumption in the absence of extremely hot weather.
A visit to the vet, even though it will cost money, can save your dog's life. Please keep us updated as what is going on with him.
Thank you for the advice. Maybe I was not clean about the fact that Skyler has been to the vet for this numerous times. I am taking him again today. However the air purifyers I am looking at are high tech and remove many many airborn problems. I do not use alot of chemicals to clean. And if I do the allergey test and the x-rays it will be about 400.00. On top of what I have spent. He is very much loved and taken very good care of.
Thank you. I replyed to the two posts and appriciate your reply.