Its been awhile..but i think he is in Slateport City, there should be a sign by his housey thing
In Slateport City, to the left of the Pokemon center.
There is a man in Slateport that will change your pokemon's name.
The name rater can be found in lavendar town.
You Can Go By The Pokemon Name Rater
Find the name rater.
The name rater is in the building on the left in the northern part of Castelia City.
when you first catch a Pokemon you can give it a nickname but if you want to change its nickname then you need to go to slateport city and talk to the nickname rater (he will change the name of any of you Pokemon that are in your party)
The name rater is in slateport above the trainers fan club. Good Luck!
You can rename pokemon when you first obtain them or ask the Name Rater to change it. The Name Rater is in Lavendar Town he's in a house below the Pokemon Center. Bring the pokemon you want to rename to the Name Rater then he will let you rename it. Keep in mind pokemon obtained through trade cannot be renamed.
You can rename a pokemon by going to the Name rater
The Name Rater is in Goldenrod City, in that purple tent thing past the train station.
Northcoast town