Think in layers. You need the color footage on a layer and the black and white kid on a layer. Then you'll need the black and white layer to slowly disappear.
In order to isolate the kid in the footage to make him black and white, you will need to shoot the kid with a chroma screen behind him so that you can key him out or you will have to resort to rotoscoping.
If the camera isn't moving, a portable green screen that a production assistant would carry behind him might work. You would just need to make sure to shoot a blank plate to cover up the green screen and production assistant! I do NOT recommend shooting the kid in a studio with a green screen because the lighting is a lot harder to match that way.
Rotoscoping might be your best bet.
ok how would i make the black and white layer slowly disappear? (thanks for the help btw)