Welcome to HCM
We understand your concerns
After going through the details I suggest that your son is simply having some sort of muscle cramps
and movement while sleep and usage of excess pillows suddenly etc are all causes behind the soreness. Children and teen aged people do sleep heavily and often develop such soreness of neck.
You can apply some sort of pain relief gels and apply hot fermentation twice or thrice and the pain should ease.
I hope I am able to answer your query. I am also available for further assistance.