Major facts that if were false would falsify the photon theory of mass.

A photon moving in a straight line is electromagnetically neutral. Its positive and negative fields balance and neutralize each other.

A photon moving in anything other than a straight line is not electromagnetically neutral. Its positive and negative fields can not be symmetrical and so cannot balance and neutralize each other. This imbalance shows up as an electric charge.

How to prove that the most fundamental constituent of the universe is the electromagnetic field.

What is the mechanism by which gravity bends the path of a photon?