The Barman works in the Port Sarim Ratpit Bar and the Varrock Ratpit bar, found in the Rat Pits beneath their respective cities. He sells stew for 20 coins and beer for 2 coins.
As he's located in the Rat Pits, you must have reached a certain stage during the quest Ratcatchers, more specifically in the part after you help The Face for Port Sarim and Hooknosed Jack for Varrock.