Clogged tear duct

My son is a month old and has goop in his left eye anytime he's been asleep for a long period.  It's a pale yellow color and crusts on his eye.  I've been cleaning it off with a damp cloth and a few minutes later he will have some yellow discharge in the corner of his eye again.  His right eye seems fine and has no discharge.  
I'm not sure if this is worth a call to the doctor; has anyone else experienced this?