Image background

[ISeek]

Hello
I took a picture of my son and have removed the background. Now I am placing a studio background in it. But the problem is that my son is sitting in the photo. How do I show that the half of the background is on the back and on half of it he is sitting.
I can put the shadow under his legs but the background seems all vertical. How do I add curve to it so that it simulates the studio background where it is scrolled down from the top and then you sit on it? (Not sure if you understood what I meant)
Attached is the background I was thinking of using.

[MiningArt]

The first image, drew two shapes and used your image for the blend/texture. Then changed their perspective. The second image is a crop of the first. I also tried one using gradients, but this method came out better.