You just stand still and wait. When he gets to you, do NOT immediately reach out to catch him. To a horse, anything sudden or unusual is dangerous so moving your hands is an attack, especially moving your hands toward his head. Staying relaxed and calm, turn sideways and stand alongside the horse's front legs with your belt buckle toward his shoulder. Now you can reach the horse's chest to scratch without moving your hand very far.
Grooming is a common language of respect and comfort among horses.
They don't do any grooming when they are afraid and if you groom in a calm way they will feel there is nothing to be afraid of. Keep your shoulders parallel to the horse's body as you scratch and groom. If you can find a place where the horse really likes being scratched, you have his attention on you. You want to captivate the horse, keep him heeding everything you do. After you're through grooming and scratching in these first lessons, just bring him back to his stall.