First of all, I duplicated the image twice, with the first one, I rebuilt the background using the clone tool and the heel tool. I then deleted all the background and half people from the 2nd copy of the image. I then improved the tone of everybody's skin by using curves and vibrancy and what not.
I deleted everything surrounding the boy's head and neck, then deleted the background from an image of a man standing in jeans and t-shirt (me) and his head. Making sure the boy's head is to scale to the body, I make sure the skin tones of me and the boy match and blend in any irregularities.
Copying the new image of the boy, I placed it on the layer underneath all the other people, put him in place, and tried getting him the correct size. I then cleaned the image of the people by blurring pixelation on the face arms and hair, deleting the blur round a few of the peoples hair, and making sure the join between them and the boy was a nice line. The tone of the boy was then adjusted to match the other people. Anything else I tell you will give some of the tricks I use away! All in all though for a none professional, this would probably take the best part of a day.