no medical professional could make an accurate assessment/diagnosis based solely on a vague description. your son needs to have this lump carefully examined by a medical professional. it could be many different things. a sebaceous cyst a reactive post-occipital lymph node. or? please encourage him to do the right thing and have it properly examined and diagnosed.
my 23 yo son developed a 2 inch lump at the base of the skull just above the hairline. the lump is tender and red and directly in the center of his head where the spine ends.