Throbbing is a vague term. Does he mean more of a pulsating, like hearing his heart beat? Throbbing usually is related to how pain or pressure manifests.

Sometimes, PCS can manifest as heightened awareness of various stimuli, such as sounds, visual images, etc. Could he have a old sensation that is now more noticeable? The brain can sometimes lose some ability to control blood flow/pressure.

Has he taken his blood pressure?
Has he tried noticing it while sitting up in a very quiet room? Even with it dark and his eyes closed?

Has he tried sleeping on his back, i.e. without his ear on his pillow? Concussions have a neck injury component. Maybe his neck is getting inflamed and causing this symptom.

Lots of questions, not many answers. Sorry.