My kid did that for about a year and he wasn't even in preschool yet! It would take him a week or 2 to get over a runny nose, only to turn around and go through a week or 2 of coughing. When I would vent to other moms about this, they all just advised me this is a good thing b/c he is building up his immunity.
Looking back I think they were right. The 3 years my kid was in preschool, he never got the flu (we get flu shots every year) even when half the kids and teachers were out, and now that he's in K he hasn't missed a day of school once since it began.
Just make sure to keep the drainage out of your son's sinuses via saline squirts and suction/blowing his nose. Limit his sugars and dairy - they make the body develop mucus. Give him a steam bath at least once a week...if you have to, squirt some saline in his nebulizer and let him breath in the steam that way. I would also up his Vit C intake, as well as adding whatever fruits and veggies you can to his diet, along w/plenty of lemon juice and garlic in what you are cooking.