L.N.
a few that come to mind from experience with my kids:
strep (this one is tricky). one of my daughters only has stomach pains and complained that her legs hurt. turned out it was strep. no fever. no cough, nothing. it was strep.
hunger (we call it tummy monster)
constipation.
call the doctor and take your son in to see what it is. always better not to ignore.
good luck