Hi J. - this would be worth a conversation with your doctor about. Kids who are not over heating at night because of room temp or being overdressed don't normally have night sweats like your son does. Be sure he is hydrating during the day because he is losing a lot of fluids at night.
Your doc will help you with possible causes but some things I would ask my doc about would be the possibility of an infection, hyperthyroidism or other hormonal imbalance, GERD, asthma, apnea or other condition that constricts the airway.
Write down if your son is having any other things come up that you might not have thought about before - things like fatigue, head aches, stomach aches, sore throat, dry cough, wheezing. You might also observe your son while he is sleeping to see if he appears restless, snores, or shows signs of apnea. You could also test some things like GERD by adding another pillow so he sleeps elevated and giving him a TUMs before bed.
I hope you find your answer!
EDIT: something I just thought of - do you have a plastic cover on his mattress or an allergenic cover on his pillow?