In general, the only indication for regular endoscopy in someone with GERD is in the setting of Barret's metaplasia, a condition which potentially evolves into adenocarcinoma. Theoretically, you can detect adenocarcinoma at an early stage and improve the chances of survival. In this setting, regular endoscopy with survelliance biopsies is performed in a systematic fashion. Should cancer be detected, surgery and possibly adjunctive chemo, would likely be recommended, and patients should know that once they embark on this sort of screening protocol.
Ideally, your father's endoscopist would be able to inform him in some way shape or form what he is paying for, since we have this free market system of healthcare. (ex: "for every 250 patients with Barret's metaplasia whom I scope in a given year, I detect 1 additional early stage cancer that wouldn't have been discovered otherwise")(not an actual representation of real data). If the specialist is unwilling or incapable (by being unfamiliar with the medical literature) of doing so, I'd recommend finding one who can.
Preventive medicine in lay terms often refers to disease screening: early detection of diseases which can be more effectively treated of discovered sooner than later. Most screening tests should increase detection rates AND show a mortality benefit among the population screened. This is the case with mammography and colonoscopy for instance. People who have these studies done over age 50, at certain recommended intervals, statistically live longer than people who don't. Both studies generally are thought to be cost effective. So I would say yes, preventive medicine as you describe it is possible and is occurring all the time.