What you need is a "prop" - some detail that being combined with your image will force the viewer to believe the boy is standing firmly on the ground.
I would recommend these steps:
Add some adjustment layer, say Levels. Leave all the parameters at their default values. Just click OK. Add mask to Levels layer, fill it with black. Now add Bevel and Emboss style. Take small white brush and carefully draw on the mask right around the boy's feet.
You should see the illusion that boy's feet are pressed firmly into the sand.
Now play around with Bevel and Emboss parameters to find the ones that make the right thing. Pay much attention to Light angle to make it the same as on the image.
Try to add some noise to your layer mask brush strokes to get more natural view.
Try to add some noise to the sand around boy's feet(take the selection from Levels layer mask). This will help viewer's eye "cling" to the details. Or even add some detail like the wet sand under boy's foot. The one like under elephant leg.
Won't hurt to cast some shadow from the boy. Again make slight distortions to it, so that it falls on the sand the right way.
Do something with the boy's pants. Select them and use Image->Adjustments->Shadow/Highlights...
This is all test and probe way, I just showed the right direction...