The main path of Opelucid City is a big circle around the city. Going counter clockwise from the Pokemon Center leads to a house. Inside is a guy on the top floor surrounded by Tympole. He wants to see your Tympole.-Source
So he is located in Opelucid City, look around for him you should find him.
The last important house may not be of much use to you right now. Head to a building just North of the Pokemon Center. A man with a ton of Tympoles will be on the second floor. If you happen to have a Tympole, his Tympoles will line up and sing for you. The only way to get a Tympole is to breed a Palpitoad or trade for one.-Source
You get no items, just a show.