D.W.
It could be allergies. if you have any animals he could be allergic to them. also if they air is dry you need to run a cool mist humidifier. have it running before he goes to bed so the air is not dry. I run a cool mist humidifier and i have a air purifier in the room. may be take a warm washcloth to his eyes before he goes to sleep. if it persists take him to the pediatrician.