The most likely explanation is that he has recurrent infection at the site. I suspect that there is a stitch or other foreign body there.
It is unlikely to be related to an artery or vein (like jugular vein), since your father has no bleeding, and has pain (which suggests infection).
He should benefit from a small surgical procedure to check for a source of infection. Recommend to your doctors that the procedure should be done under general anesthesia rather than local. This will give them better control if they find something unexpected.