It is possible he is having migraine headaches. Migraine headaches do not alway have other symptoms such as tingling in the limb before the headache appears, depression, noises in the ear, scintillating lights before the eyes.
Throbbing pain most often in the same location, frequsntly radiating to or from an eye, nausea, vomiting, intolerance for light or sound. Migraine is one of the major headaches, in the same seriousness category as cluster headache, high blood pressure, Encephalitis, uremia, whiplask, and Pheochromocytoma.
Yes, headache is the most common symptoms to man. Almost anydisorder can initiate an attack.
To list the many types, causes, symptoms of headaches that may help you identify the nature of headaches your friend may be having difficult with would take days. So the best way to prevent "blindly" guessing, the right choice to do is not worry about it if he is not going to tell you.
Hope this answers your question!
REFERENCE(S)
1. National headache Foundation, 2006.
National Headache Foundation: Select