Here's a good article on things to watch for:
http://pets.webmd.com/dogs/dog-sneezingNow, if he has a cold, he has to have caught it somewhere - the most likely place is a boarding kennel, clinic, etc. Has he recently visited a vet or kennel? If so, a cold is a possibllity. But a cold in a dog usually does also include a fever and/or lethargy. So if he's still his happy, energetic self and you don't believe he has a fever, I'm more likely to believe he has some type of allergic reaction. And it might not be an allergy, but an irritant. If they are working and he's breathing in the dust, etc - that might be doing it.
Watch him closely for signs of it being anything else. And don't overdo the benadryl as if it is a cold, that might mask the symptoms. Give it only as needed, and see if it helps. You just might have to monitor when he goes out while that works going on.
Hugs to Spinner.
Beth
“Dare to live the life you have dreamed for yourself. Go forward and make your dreams come true.” ~ Ralph Waldo Emerson ~
"He who treasures the small things in life has found the path to true happiness"