If a child is unconscious, the first step is to check his mouth for anything blocking the airway. This could include his tongue, food, vomit or blood.
If a blockage is found, roll him onto his side, keeping his top leg bent as shown. This is known as the recovery position. Clear any blockages using your fingers, then check for breathing.
If no blockage is found, roll the child onto his back and check for breathing. Listen for the sound of the breath, look for movements of the chest, or feel for the breath on your cheek.