runny eyes

My golden lab has never had issues with his eyes. A few months ago we started going to the dog park (fun!!) but he developed an infection in one eye about a week later. Went to the vet who gave me eye drops. The one eye cleared up but then both started weeping. Went through all the drops, and now he still has eye gunk every morning and I'm constantly wiping little yellow eye boogers.

He was a country dog and is now spending lots of time in the city (boyfriend w/apartment) Could it be from car exhaust? Allergies? The eyes are not red or irritated, but every time I look at him I have to wipe his eyes. It's driving me nuts!

Advice anyone?