Create a new layer, fill that layer with black, and set the layer mode to "color" this will remove all color from the layer, and will effectively convert color to black and white on the fly.
add layer mask, set the layer mask to white. This will let us change parts of the black and white photo to color.
use the path tool, take your time, and trace noah.
In the paths tab, right-click on the path you've created, and click "path to selection." This will select noah.
using the bucket tool, fill the selection with black. Make sure that you're on the layer mask, not the image when you fill in.
Using the brush tool, and setting the brush to the color white, brush off the excess color from the grass near his shoes.