Most men that wears those thing says that it must be worn in the left ear. But personally, I do not agree in the idea that men should wear earrings because if we go back to ancient history, I am referring to the bible, only male slaves wore those thing.
Usually, men wear an earring on the left side. It is said that men who wear an earring on the left side are straight, but this might as well be a misconception. According to the newest trend men can wear earrings on both sides.