Hello Dave, I'm Dr. Deb.
I recently came online and see that your question about Sjos hasn't been answered. I'm so sorry that you've had to wait for a response,but if you still need assistance, I'd like to help if I can.
There actually could be a number of different reasons for the symptom that you are seeing; there wouldn't be just one explanation. The following are possible explanations for sneezing in a dog this age:
1. Upper respiratory infection or sinus infection with the discharge a green or yellow color. Antibiotics are frequently dispensed and improvement is seen within a short period of time.
2. Allergies although these problems usually develop in younger dogs. Still possible if Sjos has been exposed to something new at your house that he wasn't in his previous environment.
Anti-histamines such as Diphenhydramine (at a dose of 1 mg/lb twice a day) may be useful/helpful although sedation can be a side effect.
3. Nasal mites can cause sneezing but may be difficult to find. There isn't usually a color to the discharge, though; the dogs are just sneezing.
Treatment is fairly easy, though, with Ivermectin given every week for three doses.
4. Nasal polyps can cause dogs to sneeze although, again, it may be difficult to prove without additional testing such as rhinoscopy and/or an MRI.
5. Fungal infections can cause dogs to sneeze but usually the discharge isn't clear; it's usually more yellow/mucoid and is not terribly common but it depends on where you live.
6. Infected tooth/oronasal fistula is also a cause but less likely if Sjos recently had any dental work done.
7. I always worry about a nasal mass when older dogs start to sneeze and this is a new development.
These dogs can begin with sneezing and the discharge may be clear but will often develop into a mucoid or bloody color over time...much depends on the location of the mass though. If it is in the back of the nose, then the discharge will flow down the back of the throat ....similar to post-nasal drip in humans. If the mass is located more towards the end of the nose, then there will be less drainage down the throat but more discharge from the nose.
Diagnosis is usually made with a combination of MRI and rhinoscopy/biopsy.
If Sjos has only been sneezing for a short period of time but is otherwise acting normally (as you indicate), then a trial of Diphenhydramine could be tried.
Otherwise, if this were my patient, I'd prescribe antibiotics on the off chance that an upper respiratory infection is present.
I hope this helps although, again, my apologies for the delayed reply. Deb