Cats can get colds. It's usually caused by feline herpes (which I think something like 80% of all shelter/stray cats are infected with). It can be dangerous to young kittens but for adult cats it usually just causes occasional cold symptoms.
Athena used to sneeze a lot (complete with "projectile snot"). It seems to have improved dramatically as she's gotten older.
Stress can trigger a flare-up.
If it's just mild occasional sneezing, I wouldn't worry too much. If it seems severe or he seems uncomfortable, then you may want to talk to a vet about what you can do to help alleviate his symptoms. Lysine can help with these flare-ups, but I don't personally know the exact dosage so someone else with more experience in using it will hopefully have advice.
Yes and yes. The majority of kitty colds are caused by the herpes virus, most cats have been exposed at one time or another. They never get rid of it, it lies dormant in the body and can become active in times of stress.
If sneezing is the only symptom, it could just as easily be be an allergic response and not a cold coming on. Just monitor him...if he becomes lethargic, feverish, stops eating, vomits, diarrhea or has yellow or greenish discharge from eyes or nose...then it's time for the vet.
So he's on l-lysine after a call to the vet. I got him from the shelter and he had URIs while he was there. I'm betting he has herpes. He's sleeping a bit more than normal but I get sleepy when I get a cold. How much lethargy would be something to be concerned about?