I'm not a physicist, but, why should this be fairly easy to disprove? The person proposing this theory seems relatively informed and obviously still published,

I'm not a physicist, but, why should this be fairly easy to disprove? The person proposing this theory seems relatively informed and obviously still published, so it's obviously not trivial. My (naive) assumption reading the article was that he wasn't proposing that a photon loses velocity over long distances, but that the value C was not constant on a multi-billion year time horizon. I don't see an easy way to prove that it is not changing if the rate of change is functionally unobservable. But, maybe I am way too uniformed to speculate on this.