Even though he wore a mouth guard, if he grinds his teeth, he is doing *some* amount of damage to his TMJ. I think the oral surgeon was overgeneralizing. Men who brux are not immune to the consequences of bruxing! Get a second opinion. I found that when I bruxed a lot/more forcefully, I developed pain in my jaw, cheek, and into my temple, and depending, it could have gone elsewhere on my head/neck.
Even though he is generally a relaxed guy, he might be developing headaches out of a psychosomatic fear of a brain tumor. If all other physical ailments are ruled out, counseling and/or hypnotherapy would probably resolve this.
Also, it sounds like he is suppressing stress. If you grind your teeth, you're doing it out of stress. So, to prevent worse problems from the bruxing (cracked teeth, TMJ disorder, arthritis, etc.), and to sort through the emotional cause of the stress and bruxing, he would benefit best from a combination of therapies. Counseling is great, but to stop the actual habit of bruxing, he might consider biofeedback or acupuncture.
Though I'm pretty sure this is the cause, here are two more questions for you:
How much water is he drinking? Dehydration *will* cause headaches. If it is the cause, here, it might be a couple of days before he gets relief, so don't let him give up if he's not feeling better after a few hours.
Has he seen a chiropractor? They can work wonders. One or two sessions might be all he needs, if the source of the problem is chiropractic.
Again, I'm pretty sure it's the teeth grinding, so he should see a dentist, first thing, and look into the biofeedback and acupuncture.
L.