The aircraft landed on Runway (RWY) 14 at EIIM and decelerated to taxi speed on the runway. The Pilot felt an initial slight downward tilting movement to his port side. He turned off the runway onto a taxiway, and almost immediately felt a further rapid drop of the port wing. The Pilot halted the aircraft, shut down the engine and he and his passenger exited the aircraft. Subsequently, it was found that a nut had come loose and separated from the port landing gear inner securing bolt, allowing the port landing gear to partially collapse. The Investigation makes two Safety Recommendations.