He predicted the photoelectric effect, and later experiments confirmed his analysis. Much later, Louis de Broglie figured that if light is or acts like a particle, maybe the electron is or acts like a wave. His theory turned out to be correct, and soon after, quantum mechanics was in full bloom.
Light energy is always multiples of the photon energy.
You cannot have a fraction of a photon. So this is particle-like behavior. Whether you say it is a particle or acts like a particle is a matter of style. Light has both wave and particle characteristics. In that sense, the photon has zero mass.