Well, he appears in a lot of places, but only after you arrive in Goldenrod.
After the player arrives in Goldenrod City, Cameron will periodically be found in both Kanto and Johto on varying days of the week, and in a few cases, at different times. He will take pictures of the player's party and the surrounding area. Cameron's house is in Cianwood City, which has an automatic camera beside it for use at all times; however, Cameron cannot be found here.
A full list of his locations can be found here, as well as the source for the above paragraph.