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 you find a blockage, roll him onto his side, keeping his top leg bent. This is the recovery position. Clear blockages with your fingers, then check for breathing.
If you find no blockages, check for breathing and look for chest movements. Listen for breathing sounds, or feel for breath on your cheek.