I think his body was just really shocked, since bettas are cold blooded they can't adapt to temperature change as quickly as us. In my fish physiology class they general rule is to increase or decrease water temperature slowly to allow their body to adapt to the temperature. So the sudden shock of hot water probably only shocked him since it was so quick. He could have mild mucus, and scale damage from falling into the sink and having the hot water pour from the tap onto him. Like kelly said you should keep the water very clean to prevent bacterial infection or fungus formation.
he'll probably be fine if he was only exposed to the hot water for a second or two