Hi SS---My first thought is that the air might be dry and so you might try a humidifier at night. I can't remember the name, but there is also an OTC product that will help to moisturize nasal passages. Just keep an eye on him. I doubt is is anything serious, just a byproduct of his cold and runny nose. Have him drink lots of water to hydrate those mucous membranes.
Feed him LOTS of extra fruits and veggies. It will help him to recover faster and, when you continue to eat that way, prevent him from getting colds in the future. Good luck. D.